Oddly, no strong feelings about those, but I am a my-ess-queue-el person. I can't claim to be consistent, but filling in vowel-less acronyms feels wrong. I think that's the difference - ctrl and fn are abbreviations, not acronyms. That might be the first time I've expressed it that way, actually. TIL, chiefly about my own thought processes.
I do believe SQL is not an acronym, but an initialism, precisely when it's spoken as you say it: initials rather than a word. It's when the string of letters is pronounced as a word that "acronym" is accurate. So SQL is either, and thus both...
But yeah, the filling in is odd unless, as in the case of ctl/ctrl, knowledge of the original guides us into lossless compression.
