I believe this おめっち is singular "you" because this person is speaking to one person in front of him. おめ corresponds to お前.
っち is probably a suffix explained here (oh, it's your question). おめっち is not common but I sometimes hear おれっち/おらっち, which means "I" rather than "we".
And this からかわれている is not "から + 変われている" but the passive form of からかう. から meaning "because of" or "out of" follows a noun that represents an emotion (e.g., 怒りから, 焦りから), but it does not follow a personal noun.
You're being teased by Aoyama!
EDIT: Oh, according to this explanation on Wikipedia and 大辞林, っち in おれっち is etymologically 達! Still, as far as I know, it never means plural "we" today.