Layout question: Magic will ~ maximally densely tile contacts anywhere I have a contact area. If I have a very large M1 and M2 layer I have connected together (say a guard ring with a metal layer that is both M1 and M2) will there be any downside in having the whole area be a contact area too? Or will too many vias significantly increase the resistance of the M2 layer / reduce current handling or something?

I would think that many contacts would decrease the resistance... I don't see any downside.