aimiable is wrong. I suspect that it is influenced by the French word aimer, 'to like'. But that in itself is a mutation from the Latin original amare, which is the source of the English word.
It is also possibly influenced by the pronunciation. The first vowel in amiable rhymes with 'aim' IPA: /eɪm/, not with 'am'. /æm/.
OED doesn't record, even as 'erroneous' - nor as far as I can see in the quotations it adduces for the word - the spelling
aimable (nor aimiable, except in the 'alternative forms' of the 15th century) in English usage. (And Microsoft Word's Spellchecker has just marked these as 'Not in Dictionary'.) Oxford reference on-line does not record them either.
And here have I been mistaking the spelling as long as I can remember!