Why would he make enemies with them before they're gone? I think he'll just continue to strengthen his position, and when the Empire collapses, he won't make a move with his armies, he'll just strengthen Morrowind even more. When a new power rises, they'll have a great economy, a strong army and a rich nation. Seeing how versatile Dunmer are in combat, i don't think this new power will just charge in, and i can already imagine Helseth trading and stuff with them. This will make Morrowind strong, rich and with influence. And they lived happy ever after.
Make enemies with who before who's gone? I don't get what you're referring to.
And I don't think the Empire will collapse. It will be weakend, and it might loose some influence and foothold in provinces like Morrowind, Summerset Isles, Valenwood, Hammerfell and Argonia, but it will not be completly destroyed.
That's a good point about Morrowind being more prepared for possible anarchy than most provinces. At the same time, the fact that Morrowind borders several other provinces might be a challenge for them, whether it means having to deal with refugees for example, or possible future conflicts with the Nords, Argonians, etc.
Well, the only provinces they border to are Skyrim, Cyrodiil and Argonia. Skyrim consists of several bickering city-states, which would take every possibility they get to invade their neighbours, whether they lie in Skyrim, Hammerfell, High Rock, Cyrodiil or, I guess, Morrowind. These leaders and (possibly) war lords are also, to me, a few of the most plausable to reach for the Imperial Throne, focusing on that instead of invading Morrowind. Anyway, since they are not united in the same way Morrowind is, they would not be strong enough to become a serious threat to Morrowind borders.
Cyrodiil would be in the center of an Imperial Civil War, the goal of every emperor-wannabe in all of Tamriell, and the cities there would be to busy defending themselves to attack anything, anywhere. Instead, eastern Cyrodiil would be a place where Morrowind has the greatest chance of increasing it's influence, making it dependant on Morrowind trade.
Argonia is just a plague-ridden pool of Argonian tribes that just wants to be left alone. It's probably the province that would be least affected by a "succession war", since the Imperials have problems keeping in the first place. But I could imagine Helseth approving Hlaalu and Dres settling further down the border, more or less "colonizing" the area.