"é" is acute... is on word like "perché" "poiché" "affinché" "abbatté" "frappé" (even if we say it with acute accent, but it is a French word and we write it in the French way)

"è" is grave... "è" "cioè" "tè" "caffè"

"è" is used far way more often.

We use the acute accent on compounds with "che" at the end and on some words taken from French. Also sometimes in "passato remoto". Probably in some other cases as well, but not many.

No, they aren't interchangable "è=is" (the third person singular form of the verb essere = to be), while the other one (é) by itself , as far as I know, doesn't mean anything.

L' is usually used when the word ends in a vowel i think