Re: concubine
I’ve read in a book and several other places that he did marry her. Here’s a case in point:
http://www.themodernreligion.com/women/w_polyplural.htm
Likewise, it is allowed for a man to marry woman of low social class as in the case of Maria (RA), who was given to the Prophet (SAW) as a present by the ruler of Egypt. The Prophet (SAW) elevated her status by marrying her, instead of making her his slave.
It is known that maria (ra) was a copt when given as a slave to the Prophet (saws) but that she did in fact become Muslim. If he did not free her when she accepted Islam (and then married her to stay with her), this would have meant that the Prophet (saws) kept a Muslim slave.
Edit: I’ll need to check my book when I get back home, but I think it said that Maria (ra) was not given the title “Umm-ul-Muslimeen” granted to the rest of the Prophet’s wive until after she gave birth to his son Ibrahim. I think but I’m not sure, that the book said she became his wife at that point.