/.../ If you're at 0 red hearts you actually will gain a heart. /.../ Grab the dead cat, now you have 9 lives. Grab the other two items, one will kill you but now you'll have 8 lives and lots of items. As long as you can secure some soul hearts after you should be golden/.../
This was changed in Rebirth sadly. You can be reduced to 0 hp from devil deals and live and you can't take red heart deals if you don't have enough hp it won't kill you, you just run over it
Question: Does using a seed have drawbacks? Does it disable unlocking of things or win counters or stuff like that?
Yes, you can't unlock stuff using seeds, it would be too easy.