Marvel zombies would be pretty tame tbh, given that we already have the equivalent of that disease with Poxwalkers from Death Guard, who would either subsume the disease if it wasn't from Nurgle already. Similarly, Tyranids would be able to adapt to it pretty easily, while non-organic factions like Necrons would be unaffected and solve the problem through conventional means.
Unless I'm misreading your point and you're saying all of the zombie heroes/villains from the comics. Only a few of them with the more OP comic powers would make an impact and that's assuming they have access to faster than light travel. Even then their sphere of influence would be very limited, especially depending on where they are dropped on the galactic plane.
If Tyranids showed up at the start of the Heresy, it depends on how many and which Hive Fleets came up. Part of the reason why the Imperium is struggling now is because there's so many threats they have to deal with at the same time. If this is before the dropsite massacre, it could drastically change the course of history since it's very possible that Horus would parley with the Imperials or post-pone some of his plans (or adapt to try and use the Tyranids as a 3rd-party way to maul loyalist strongholds) since Horus would never allow the Tyranids to take Terra. Traitors they may be but they still have the xenophobia that was present in the Great Crusade. It would really depend on how much actual coordination and backstabbing occurs between the Imperials and the Traitors. I would assume that Horus dismisses the threat initially, as would the loyalists, until they see how vast and dangerous they are when they start consuming multiple worlds. With Primarchs and the legions at their heads, I feel like the Tyranids would be fought off but at great cost, and possibly
If Sanguinius had defeated Horus on the Vengeful Spirit, the Emperor would not have been crippled by his fight with Horus and the Imperium would not be the way it is now since it would still be largely directed by him, even if he was stuck on the Golden Throne. More importantly, the BA would not suffer the Black Rage from Sangy's death, so Death Company wouldn't be a thing anymore. Dorn wouldn't get super depressed and go through the Iron Cage thing, more Primarchs would likely have stayed, etc.