If I was on Jeopardy, and "definitions" for $1000 was "Key", my answer would be:
"What is 'extremely or crucially important', Alex."
I don't find Avengers Vol. 4 #12 to fit that definition in the slightest. I find the money-inspired opinions to be completely false.
Movies, TV shows, and speculation causes demand and inflation. Doesn't magically transform a comic into a key.
If Professor Hulk farts in Avengers 5, does the comic in which Hulk was drawn farting become a key? No, it doesn't. Will it increase in demand, and consequentially, price? Probably. It would never be a key.