"Whose cat is eating meat?"
Translation:Czyj kot je mięso?
No. When asking about the owner one should use czyj.
Yes, "czyi" is plural, and on top of that it's 'masculine personal plural', so it only works for 'groups with at least one man'.
So if the English sentence had "Whose cats" in plural, that would be "czyje" ('not masculine-personal plural' - the plural for 'anything else' than the one I mentioned before).