That many fixtures on on line, I doubt you can get away with it. It was most likely the mic cable causing the issues in the first place and who knows how long before issues pop up again. A terminator at the end of the line also won't help since the mic cable is a different impedance then the DMX and terminator.
As far as mixing and matching XLR and DMX, again, it might work, it might not. I would, however, highly recomend that you start replacing the XLR with DMX like you are trying to do. Also a DMX splitter wouldn't hurt either and probably help clean up some cable runs.
I have done gigs where literally one 25' XLR from the controller to a single fixture caused it not to work. And my boss threw a hissy and swore up and down that it couldn't be the issue. Changing the cable didn't help. For whatever reason, some sort of interference was getting picked up by the XLR and causing it to not pass proper signal.