According to the Hertz Chumash, The Pentateuch and Haftorahs, page 202, the word 'almah in the Tanakh means a young woman of marriageable age, unmarried or married. In modern Hebrew 'almah means 'miss', a title for a young unmarried woman. While this term certainly does not mean 'virgin', as major Christian Bible translations now concede (NRSV, NABRE, NJB), where is there an example of a married 'almah among the seven 'almah passages in the Tanakh (Genesis 24.43; Exodus 2.8 ; Psalm 68.26; Proverbs 30.19; Song of Songs 1.3; 6.8; Isaiah 7.14)? In the fifth century, Jerome, who translated the Tanakh from the Hebrew and was taught by Jewish teachers, wrote concerning this: '... let them [the Jews] at least show me where the word is applied to married women as well, and I will confess my ignorance' ( Against Helvidius I.32). If the clause ha-'almah harah in Isaiah 7.14 is advanced as such a proof, then the assertion of Rashi that it is future and of Malbim that it is not must be resolved.
The question has an unspoken, Christian-missionary-oriented premise. (Unfortunately a common subversion of this site, and one not acknowledged by either askers or answerers. Let us not pretend.) Namely, if an alma only means an unmarried woman, and an unmarried woman in Israelite society was naturally a virgin, then alma must mean virgin, and we arrive at a virgin birth.
Other answers address, head-on, how we might find an explicit married alma amongst the very few instances in Tanach, as if that is what is indeed required. They do a good job, but one need not prove an instance of a married alma to support the assertion given from the Hertz Chumash, namely that "the word 'almah in the Tanakh means a young woman of marriageable age, whether married or not".
There are different categories of words, some general and some specific. For example, an איש in Hebrew might mean a man, whether married or unmarried, whether a virgin or a non-virgin. It does not carry additional meaning. Meanwhile, a בתולה might well mean a young virgin. A בהמה can refer to a domesticated animal, such as a sheep, goat, or bovine, while an עז refers to a specific animal, a goat. Further, there are not macro and micro levels, but intermediate levels at which some attributes are specified and others are not.
The question is whether, based on pesukim, one can establish the word עלמה as a general young female, or a young female who is specifically unmarried, or specifically a virgin.
In Genesis 24:43, Avraham's servant is recounting the condition he imposed for identifying Yitzchak's destined bride. Earlier, in verse 14, he refers to a naara, and thus alma is a synonym of this. A naarah can be a non-virgin (see Dinah after the rape). And a naarah can be a married non-virgin (see Devarim 22). So just as naarah can be understood in the general sense, the synonym / parallel can be understood in the same general sense.
If you want to prove that it has a specific sense of non-married, then it is upon you to cite a verse from the sparse seven verses which demonstrate that this is the required meaning. Otherwise, there is no reason to impose the non-married specific meaning.
If Herz has said that alma meant "a young woman of marriageable age, with no requirement that she specifically be a brunette", then it would not be upon anyone to demonstrate that there are non-brunettes (say, redheads or blondes) among the seven verses. Similarly, he is saying that we see this general meaning, and there is no cause to specifically assert a non-married meaning.
The OP asks (A)... "Where is there an example of a married 'almah' among the 'almah' passages in the Tanakh...?"
Answer: Proverbs 30: 18-20 (18)" There are three things that are concealed from me, and a fourth, that I do not know; (19) The way of the eagle in the heavens, the way of a serpent on a rock, the way of a ship in the heart of the sea, and the way of a man with a young woman ("Almah"). (20) So is the way of an adulterous woman; she eats and wipes her mouth, and she says, "I have committed no sin."
In the above Passage from the Tanakh, Proverbs gives three examples of things which are "concealed" in the doings of this world.
1) An eagle's path in the heavens 2) A serpent's trail on a rock 3) The way of a ship through water
Each of these things have something in common. They leave no trace after a short time. A minute later, an observer can see no evidence that an eagle, snake, or ship, have passed through.
Proverbs then picks a 4th example similar to the three before it. The "way" of a man with an "almah" (young woman). This "way" in this context, is a euphemism for physical intercourse between the man and woman. After a short time, there is no trace in the woman, that a man has been there with her. (similar to the eagle, snake, and ship on their respective "ways")
Verse 20 then continues to refer to this same 4th case of the "Almah" by calling her an "adulterous woman". The subject euphemism (of intercourse) is also continued with the words "she eats and wipes her mouth". So the verse is referring to the same case number 4 above.
The style of the verses 18-20 depict the 4 cardinal elements of fire, water, air, and earth in the 4 cases. (eagle:air, snake:earth, ship:water, and "love": fire) Therefore, verse 19 and 20 are the same case of fire and there is no separate 5th case in verse 20. So the "Almah" of verse 19 and the adulteress of verse 20 is the same woman.
(All of this is agreed to by the Malbim in his commentary to Proverbs 30:18-20; namely that the 4th case of the "Almah" and adulterous woman are one and the same, and that it is all a parable about a particular woman who betrayed her beloved king by cheating on him and trying to hide the fact (the parable used is based on the 4 elements). Malbim in a separate commentary called Be'ur HaMilim to Proverbs 30:19, says that the word "Almah" here means a non-virgin woman.
Also see Ibn-Ezra to verse 30:19 - "with a young woman: when he has illicit relations with her, he does not fear G-d when he violates the commandment of "Do not commit adultery"..."
A similar approach to the parable and assumption that the woman in verse 19 and 20 are the same, is found in Metzudas David to 30:18-20, Metzudas Tzion also identifies "Almah" here and in Isaiah 7:14 as a non-virgin as well.)
Now we all know that only a married woman can be accused of adultery. A single girl who does this might be labeled promiscuous, but not an adulteress. (She is also not a virgin, since it would be easy to make a physical examination and see she has lost her virginity. Therefore the verse is speaking of a non-virgin "Almah" who would remain physically the same before and after intercourse; and hence her actions would be undetectable like the ways of the eagle, snake, and ship.)
Therefore, Proverbs 30: 18-20, is an example of the Tanakh using the word "Almah" to refer to a married woman (and certainly not a virgin).
The OP asks (B) : Is it possible for the Tanakh to refer to Isaiah's wife as an "Almah" and as the mother of Shearyashuv; if "almah" by definition, only refers to a woman who has never had children (according to Rashi and Ibn Ezra)?
According to Rashi: Shear-Yashuv is not a real person or a son of Isaiah.
Rashi: Isaiah 7:3 .."and Shear-Yashuv your son..": The small remnant that will return to Me through you, and they are like your sons.
Here, Rashi translates "your son Shear-Yashuv, as a metaphor.
So, according to Rashi, Imannuel of Isaiah 7:14 can be a son of Isaiah and his wife, who has not yet had a child in her life. This is because, according to Rashi, Isaiah's wife is not in fact the mother of a previous son named Shear-Yashuv at all.
According to Ibn-Ezra:
Ibn-Ezra does in fact believe that Shear-Yashuv is a real son of Isaiah and his wife. However, he does not say that the mother of Imannuel in 7:14 is the same mother of Shear-Yashuv in verse 7:3. So, it is possible that according to Ibn-Ezra, Isaiah had two wives. The second wife would be giving birth to her first child, Imannuel.
Alternatively, we can say from Ibn-Ezra's commentary on 7:14, that he does not agree with the definition that an "almah" must have never given birth, since his only criteria for an "almah" is that the woman be young in years.
Ibn-Ezra: "...There is nothing in the definition (of "Almah") except as it relates to age..."
Your question B answers question A. Thus
הָעַלְמָה (the young woman) was married but Rashi say was not yet pregnant with her first child.
That is Rashi and Ibn Ezra point to this pasuk to show that even if she was not yet pregnant, she was married to Yeshaya. Radak says that it was the young wife of Achaz
Since the young woman was known to the king and was going to be involved in a sign, then she would not become pregnant while unmarried.
Those who translate the verse to mean is currently pregnant obviously state that the word
הָעַלְמָהcannot in this verse mean unmarried. Additionally, they disagree with Rashi as to meaning that she will become pregnant.
But according to the Hebrew and Aramaic Lexicon of the Old Testament, a leading scientific lexicon, an 'almah is 'a young woman (until the birth of a first child).'
Please explain fully. If the lexicon is correct, then the speculations of Ibn Ezra and Rashi have no sound lexicographical basis.
Note that Rashi and Ibn Ezra are stating that this would be her first child and that he would be born during the coming year. Thus, they do fit with your lexicographical citation.
That is, Rashi Yeshaya 7:3 says that it was not his actual son but
and Shear-Yashuv your son: The small remnant that will return to Me through you, and they are like your sons.
Of course, Radak who says that the the young woman is the wife of Achaz does not have to say this.
Ibn Ezra cites Mishlei 30:19 to show that the word cannot be virgin as
the way of a man with a young woman.
is given as one of those things that cannot be detected (after a short time). If she was a virgin, one could tell that she is no longer a virgin.
This also shows that it can mean a married or unmarried woman.
is with child: This is actually the future, as we find concerning Manoah’s wife, that the angel said to her (Judges 13:3): “And you shall conceive and bear a son,” and it is written, “Behold, you are with child and shall bear a son.”
the young woman: My wife will conceive this year. This was the fourth year of Ahaz.
This means that she was married, but had not yet conceived, but was about to in the near future.
Art Scroll Tanach translates it in accordance with Rashi (will become pregnant).