It sounds like you're kinda expecting the whole thing to be done in one fell swoop, which I highly doubt the story would do.
Sure, Nova Imperium have blasted on about how they want to topple the Emperor and put a new one in and have a whole bunch of demands.... when the eventual conflict-CG flashpoint happens, if Nova would come out on top I'd doubt ALD would be overthrown the next week.
What's more likely is it would legitimise them as an entity, resulting in other Imperial entities demonstrating that support, and serving as a more solid base to continue their work against the current Imperial establishment.
From a game-mechanic perspective to represent that as a story, it'd make more sense to, if Nova Imperium won the conflict, establish them as an Imperial Powerplay Power, but have them function more like an Independent Power in terms of how they interact with the other powers.
Not saying that's a great way to go, because I think Powerplay was a missed mark as soon as it started, but to use what we've got, that's the most legitimate outcome.
Bottom line, coming back to my previous post, it'll only be "another Emperor's Dawn" if players let it go that way.