>>5928
>Unless you introduce a 4D chess player

>>5929
>Or do you want a different player

Given the idea of the argument between the different universes' Celestias (but then my problem is, if canon Celestia offers amnesty, is Forest Fire's basket of feelz heavy enough to force him back to his prior pains? What if he could bring just Autumn Breeze through with him?)

I think the resolution needs to be, Fred/Forest Fire goes off on a quest to stop the 3rd party.
Further, the Fred we meet in the opening chapters is brilliant in certain areas but focuses that effort on being just altruistic enough to comfortably remain a narcissistic shut-in (hmm. where did I get that image from)
In order to force Fred to grow, he needs to seek out aid, accept both the help he's offered and the strings that go with it, and he needs to put everything of his own at risk for someone else's benefit.
IOW Fred needs to make alliances with the HLF-rejects, the slowly-building earth pony rising, and even Luna to rip open the worlds for him to travel and kill the invading force (machine? Maybe its a spell engine, DnD style, that flopped off its axis and is screwing with Tyrantlestia's empire-devour spells).

The deus ex machina is clearly a frindship-magic blocking ring. It would let him cast spells up through mid-unicorn energy levels but also make him functionally invisible even to earth ponies.
Hardmode: he gets one mid-story, and needs to use it to sneak through the castle where Celestia keeps the only copy of her 'twist universe path' spell that she casts every four to six centuries. The castle is already rife with multiverse holes and in most other worlds that metal acts as anti-magic, so Forest will need to rip the ring off periodically, keep it handy, and 'feel' when to slip it back on before the butler on the outside of the door figures out there's an extra pony in the bedroom for some reason.