I'm sure this has been discussed before, but does his ability still take effect if he dies at the same time? For example, if I played Life's Finale?
Yes it would bring back all other non-humans.
Would his ability keep my 1 toughness creatures alive if I have a {heartless summoning} out?
Yes(except if they are humans). Both the +1/+1 and the -1/-1 apply at the same time
Ol, that's what ibthought. Gonna make a heartless/undying deck : )