Can you chain to a summon to keep a creature face down? Example: your opponent has a creature face down and says they are going to flip summon say, stealth bird, can you in chain activate nightmare and keep it face down after they have declared what is they are flipping? Related question, when someone sets a monster can try to activate light of intervention after they have already set it and say they cant set monsters, wouldnt it take effect after and not touch the one you already set.

for your first question, no. you can't chain to a summon. you can only respond to one, so nightmare won't keep them face down.

if you play a card after it was already set, then it would affect all cards played after that point.