From an in-universe perspective did Darkseid ever stand a remote chance of winning and forcing the entire universe to be what he wanted it to be? Or would the Presence, a 5-d imp or Specter change reality so Darkseid is doomed to failure no matter what? Would they write the universe off as a failure, destroy it, and start over?

Presence generally is somewhat apathetic about the universe. It tossed together a world with the Justice League, the Green Lantern Corps, and innumerable other wonders. It gave them everything they needed to stop Darkseid. If they fail, then they've failed.

5D Imps, at least the ones that have any interest in the 3rd Dimension such as Mxyzptlk or Bat-Mite, generally are not very brave. They're reality warpers of the highest level, taking a trip to the 3rd dimension and screwing around is basically a vacation. Think of it as their spring break. Sure, it'd be a bummer for Darkseid to take over, but if your vacation spot was undergoing a volcanic eruption, would you want to drop by and say hello?

Even if they wanted to try and fix things, think about the risks: if they hear the Anti-Life Equation, Darkseid will have all of their power at his disposal. He will be truly unstoppable.

As for the Spectre... I want to say Darkseid operates outsides his jurisdiction, though I can't rightly say why. Maybe it has to do with Darkseid being an anthropomorphic personification? You don't see the Spectre going after Dream or the other Endless.

No...Darkseid did in part succeed in obtaining fragments of the anti-life equation. Orion has actually used it in full as well, so I wouldn't say it was an impossibility.

The Presence is watching over infinite dimensions and universes, He really doesn't care that much.

The Spectre's human host would most likely have been corrupted, thus negating that.

The Imps would have been corrupted as well, upon entering the 3rd Dimension.

Anti-Life > 5D tech