It is, check the OED.
Older but not necessarily wiser. Been online since 1970s BBS
It is, check the OED.
Look up Egg Theory. It will blow your mind.
Basis becomes Bases Crisis becomes Crises Oasis Oases Echolalia Echolalies Etc Generally the vowel becomes an e
Illinois Illinoes
It’s because the im isn’t a prefix but part of the word. (It was originally spelt empeche)