Assuming it hasn't been retconned, the War in Heaven was said to span several galaxies. It's entirely possible that the extragalactic Orks are leftovers from the war, and in other galaxies there are abandoned webway gates, Old One skeletons, sleeping necrons and just shit tons of Ork Empires.
Yeah, nah. Lore keeps cropping up saying it was on Universal scale (mostly because things sound cool), but it most probably was just confined to Milky Way. A lot of things stop making sense if it was over multiple Galaxies. The thing here though is again - everything is canon, nothing is true. If some Eldar bastard goes to humans "oh yeah War in Heaven was multiple Galaxies, that's how cool we are, duh" then there is a pretty high chance he's lying or just insane.
That's pretty much 40k all around.
That's why we're here, trying to figure out how the fuck does it work and theories are a major part of the fun. It's not like you can even expect universe with fuckton of books and other media, by fuckton of authors to be very internally consistent. Nobody really does that.
Then it should be possible to webway to another galaxy, which should be no more boned than the Milky Way, since the Webway itself isn't just Daemons in a tube -yet-. Less boned even, since Big E isn't there to be our Guiding Light and Savior.
I don't think so. If War in Heaven spanned multiple Galaxies, then the others are just as boned, if not worse. You might get away from the local variety of Chaos Gods, but you'd run into Enslavers or whatever they had down there. Also, as far as webway integrity, the breach Magnus caused is really just one of many. The only real problem is that it opens up in between Webway and realspace, which is a problem for Terra, but not so much for Eldar, they'd just seal it off and continue to do fetish sex in their giant BDSM basement.
Those poor Orks in the void. They're stuck floating around in big empty space with nothing to do, where they're lucky if they manage to collide into other Orks for a scrap every now and then. Sure, I guess their ships and roks will be nonstop internal fights broken up only by periods of silence after everyone's been killed and before the new spores grow, but I'm sure the Orks would prefer a little variety in their fighting. I feel for them.
Then again... I like to think that they're actually in the path of the main 'Nid armada - the only thing stopping said armada from reaching the galaxy, even, just an eternal bug-stomping Valhalla for these poor lost Orks who have no idea they're saving the Milky Way.
I'm pretty sure they're too spread out to be actually dangerous to Nids and I guess they survive by fighting each other, yeah. It did say about systems worth of trash that Orks live on, so yeah, it's not like they're really confined to extremely small spaces.