I propose to mitigate the Might penalty to source quality by five times the strength of the silver (mind) cord.
My dark munchkiny side always saw the silver cord as the least interesting, to a magus, of the three when compared to supernatural botch reductions and bonus to aging rolls. That way, it would be satisfied.
Other options to consider could be three times the sum of cord strength (if you think they are balanced as it is). Or you might want to involve Magic Lore in there somehow. Or to allow the bond to store memories (like for immortal magi) counting any vis spent on the bond as storing 5 xp. Or to allow that vis to be used up for penalty mitigation (almost the same thing).
Immortality issues only concern magic beings which are not naturally immortal (Age Rapidly flaw). For those, the bond just breaks, a psychological trauma which may lead to death, but not a cause of sudden death. Depending on what model what chosen above, the learning acquired during time spent as familiar may be lost. Or not. YMMV.