We can get a bit confused by the terminology. There is ultimately no word for "wife", rather simply "woman". A woman belonging to a man is translated as "wife" in modern English (in old English the word "wife" also just meant woman, as in Hebrew). So, a woman a man is sleeping with is his woman. Whether she's a slave, or came in free, she's his woman. Those details will affect aspects of the relationship (most critically inheritence), but ultimately if she's his woman, she's his woman. Ie, his "wife" in translation. All are therefore wives. It's not some complex process whereby a bondmaid is "converted to a wife". She just is as soon as he starts sleeping with her, because that's just the word for it.
Because the death penalty only applied to a wife who wilfully committed adultery. In the case of rape, only the man got the death penalty. Rape being defined as when the woman cried out for help, with the benefit of the doubt given to the woman (if it happened away from people, it is assumed she cried out for help even though nobody heard, and therefore only the man died). In this case, it is very clear that Absalom chose to sleep with them all to prove a point, not because the women were in any way attracted to him. What he did was rape - and those many witnesses would have attested to that. So only Absalom would get the death penalty, and he was already dead.
Note that I take this from Deuteronomy 22:22-27, and technically in this passage the rape provision is only applied to a betrothed woman. However I think it is entirely reasonable to extend it to marriage also in the light of God's character. To apply v22 completely legalistically even in the case of rape would be insane.
Where in scripture does it say that they were raped? For all we know they slept with him willingly thinking he would be the next king. Where does scripture say that a woman who is raped not entitled to sex from her husband? Again these are some pretty big problems if you assume that concubines are wives. Also prostitution was legal in Israel a man who slept with a prostitute did not make them married.