You address several rules. If there is in the list of foods, one of them from the 5 species of cereals, it is the Ikar, the main, but there are exceptions. If this food function is to sweat the damaging effect of an other food, or is only to help against suffocation, it doesn't require Beracha. The rule of 5 species of cereals is a part of the general rule of Ykar and tafel, essentially and contingency.
- Mishna brachot (in gemara 44a) taught the principle of ykar and tafel. (By the way, from this rule we generally apply in Berachot rules of rov (majority), because generally the main food is in greater amount.)
הביאו לפניו מליח תחלה ופת עמו - מברך על המליח ופוטר את הפת, שהפת טפלה לו.
זה הכלל: כל שהוא עיקר ועמו טפלה - מברך על העיקר ופוטר את הטפלה.
If salted food is set before him and bread with it, he says a blessing over the salted food and this serves for the bread, since the bread is only subsidiary to it. this is the general principle: whenever with one kind of food another is taken as subsidiary, a benediction is said over the principal kind and this serves for the subsidiary.
- Concept of lelafet et hapat (a food made to dip the bread in this food) (gemara 41b). The rule is that foods that are eaten with a piece of bread are tafel to the bread.
אמר רב פפא הלכתא דברים הבאים מחמת הסעודה בתוך הסעודה אין טעונים ברכה לא לפניהם ולא לאחריהם
- Gemara regarding the rule of 5 species of cereals, addresses a mixture or a combination of essential components and an accompanying components. The ykar of the mixture is not necessarily the component that is in greater amount. If one of the components is one of 5 cereals specie, it's the ykar.
דרב ושמואל דאמרי תרוייהו כל שיש בו מחמשת המינין מברכין עליו בורא מיני מזונות
Rab and Samuel have both laid down that over anything containing an ingredient from the five species [of cereals] the blessing is 'who createst all kinds of foods'.
Apparently the Mishna in 44a is an exception to the second rule, because the bread is from the 5 cereals.But in reality the Mishna shows the deep sense of the rule of Rav and Shmuel. (from the op-cit Mishna 44a). When the food eaten with a piece of bread is what you want to eat, but you cannot eat it alone because it's too strong, e. g. too salted. In this case, the bread comes as an attenuating food and is Tafel.
Tosfot says that the case of the Mishna comes to teach that bread is not always the essential food even if you eat the second food with the bread (as a sandwich for instance).
ואף על גב דאשמעינן דפת פוטר את הפרפרת איצטריך לאשמעינן
דפעמים דפת טפל
- The concept of Chashuv is generally used when two foods with the same beracha, to decide on what to bless.
So we explained some principles regarding ykar and tafel, you addressed in the text of the question.
- But your case address the din kdima, precedence, of Brachot. A case in which you need anyway to make 2 Berachot, on what have you to begin. Gemara 41a take an example of bracha Haadama and bracha Haets. There is an opinion in Gemara that such a case is in machloket between Rabbi Yehuda and Rabanan in Mishna (the Mishna is in folio 40b on Gemara). Rabbi Yehuda says to begin by min Shiva (the 7 species quoted in Tora in the shevach of Erets Israel), and Rabanan say to begin by the favorite. In halacha OC 211.1 there are several opinions and a lot of cases are discussed.
For the Kdima, e.g. between Shehakol and Mezonot, the Bracha of Mezonot is more specific, and Cake is from the 7 species, so, it's before Shehakol. Hamotsi for Bread is before all. See the whole siman 211 in SA OC
All this if you want to eat the both at the same time. But if there is an order in the art of eating, this order is prominent.
It's normally to eat the Sandwich before.
- Regarding the after Bracha, Beracha Acharona, if we are not in situation of ykar and Tafel, that globalize the eatingin one Beracha Acharona, the question is if there is a notion of Seuda. If there is a Kviut Seuda, the Beracha Acharona is Birkat Hamazon. If not, there are two Berachot Acharonot anyway.