: 343 has all but incorporated AFoA into the canon.

How do you figure? What have they put in place or confirmed that was only present in FfoA?

: So here's my question: Why hasn't it been made official?

: Levi said 343 once contacted him about publishing it, so why did that fall
: through? Legal issues?

Obviously legal issues, and they'd have to pay him. Plus it would open up the floodgates of fanfiction and fan art creators wanting their work to be ascended to canon status, and asking for it to happen, and getting their hopes up only for it to (almost) never happen, and possibly trying to bribe or even threaten 343i into making their stuff official. The general stance on these things is to have all fan work always stay work, for these and other reasons (mostly a spate of legal reasons).

: I feel like Canon Fodder could be a good place to finally incorporate this
: thing into the official canon, if only Microsoft would give Grim the
: clearance to do it.

No, if they accepted it as canon there would probably some other announcement and then they'd sell it in physical form.