In answer to your first question, the Torah had decreed a term of six years for anyone who became a slave. Thus if someone stole and became a slave or became that impoverished in the first fifty years after the conquest, then the first yovel would have freed those who had become slaves during the previous six years. However, the whole point is that someone would not become a slave unless he had been forced off of his ancestral land. The yovel would return the land to him so he would no longer be poor enough to become a slave. I doubt that this loss of ancestral land would have occurred so soon.
To answer your second question:
Ever since the tribes of Reuven and Gad were exiled from their land. Rashi says on Yeshayahu 8:23
that they were exiled in the twelfth year of Ahaz. Ahaz started his reign in the year 3190 Twelve years later would be 3202. The first temple was destroyed in the year 3338 one hundred twenty eight years after that exile.
Your third question would be answered as follows:
Theoretically, without a yovel, every slave would have to serve a full six year term. The torah explicitly decrees that no eved Ivri can serve more than six years. However, we learn that the people had kept slaves past the correct term as we see in Yirmiyahu 34:8 - 11
Rashi
after King Zedekiah had made a covenant: In the seventh year of his
reign. So we learned it in Seder Olam (ch. 26): “And it came to pass
in the seventh year… men came of the elders of Judah (sic) to inquire
of the Lord” (Ezekiel 20:1). They said to Ezekiel, ‘The slave of a
priest whom his owner sold what is the ruling regarding whether he may
eat of the priest’s due?’ That is to say, because they wanted to say
that they should not be punished for infracting the commandment by
returning the slaves.
The pesukim then continue that the people then kidnapped their former slaves and re-enslaved them.
That every man should let his manservant and every man his
maidservant, a Jew and a Jewess go free, that none should hold his
Jewish brother as a slave.
Now all the princes and all the people who had entered into the
covenant hearkened that every one should let his manservant and
everyone his maidservant go free, no longer holding them in slavery;
then they obeyed and let them go.
But afterwards they turned and brought back the manservants and the
maidservants whom they had let free, and forcibly made them into
manservants and maidservants.
Hashem then sent Yirmiyahu to castigate them in Yirmiyahu 34:15 - 17
And now this day you turned and did what was right in My sight by
proclaiming liberty every man to his neighbor, and you made a covenant
before Me in the House upon which My Name is called.
But then you turned and profaned My Name, and you took back, each man
his manservant and each man his maidservant, whom you had let free to
themselves, and forced them to be manservants and maidservants to you.
Therefore, so says the Lord: You have not hearkened to Me to proclaim
freedom, every one to his brother and every one to his neighbor;
behold I proclaim freedom to you, says the Lord, to the sword, to the
pestilence, and to the famine, and I will make you an object of horror
to all the kingdoms of the earth.
Rashi
behold I proclaim freedom to you: from Me, that I am not your master
to save you, and you shall be free to the sword and to the famine.
Once the temple was destroyed four years later, the Bnai Yisrael no longer had the power to enslave anyone and the institution of slavery ended.
