This May be controversial, but I think that catchers should be allowed to become injured even if they are your only catcher left. I also think that in conjunction, teams should be allowed to take the risk and only carry 1 catcher if they want.
Agree you should have to carry 2 catchers. Not thrilled with the idea of 2nd becoming invincable while the other is out but without a minor league from which to call up replacements probably the only workable solution. I do think it should be more prominently displayed in the rules this is in place so newbies will be just as aware as vets.