I tested the 3 possible choices if the mage get into the joining or not
and in all 3 she dies
didn't test yet if you can say to him to not join your party when
you meet him but I'm pretty sure it wont help either
I need answer from some one who does have Mhairi alive or
tested every possible situation.
I also tried today not letting mage join my party and he does anyway before the boss fight. I also when offered did not let the dwarf become a warder thinking it might be because I already had a fighter each other time (him). This did not work also. Not sure what the answer is yet.