Upvote:5
Just a partial answer here to one of the questions posed in the body (not in the title):
"Is it safe to assume that such marriages of marrying off a Christian daughter resulted in their forced conversion to Islam, and that they were not allowed to remain Christian?"
It was not always the case that the wife would convert to Islam, at least when the wife was a daughter of an emperor.
Theodora Kantakouzene, born c. 1330
Theodora was one of the three daughters of Emperor John VI Kantakouzenos by his wife Irene Asanina... In January 1346, to cement her father's alliance with the rising Ottoman emirate and to prevent the Ottomans from giving their aid to the Empress-regent Anna of Savoy during the ongoing civil war, she was betrothed to the Ottoman ruler, Orhan Gazi.
...Theodora remained a Christian after her marriage, and was active in supporting the Christians living under Ottoman rule.
I have nothing to offer on how other Byzantines viewed this arrangement, but as @Spencer comments, when the emperor supports it do other Byzantines matter?
She is known to have returned to the palace in Constantinople after her husband the Sultan's death in 1362. Last we know of her she is imprisoned by Andronikos IV (reigned just 1379–81) but this is more likely to be because he usurped his own father than another factor.