That is, how would I go about converting the word "spelling" (as in the spelling of a word) to an adverb that actually sounds right in the blank of "_____-inept"?
I'm pretty sure "spellingly" isn't a word, and likewise "spellatically" doesn't sound remotely close to any actual word (I suspect in both cases that it's something to do with the fact "spelling" is a gerund, not an actual noun), and I'm not what actually does work.
As always, all help, constructive criticism, and other constructive input is welcome.