If we look at precedents, it probably will end up being a three-parter like the ATLA comics, though that in itself is not a guarantee.
As for Korra and Asami, I would agree with you that there is probably no chance at all of the two going the way of the S.S. Makorra. First because I think the relationship has a much more solid basis in terms of development than Makorra did, which makes it far, far less likely that something like that would tear them apart. The one thing that Book 2 did show was that Korra and Mako just did not click the way Korra and Asami did (although that was done in a very slipshod way IMO, like a number of other things in Bk2). That, of course, is just my opinion, but the bottom line is that It is IMO almost impossible for something like that to happen with Korra and Asami.
I also agree though that at least some tension is good for developing a relationship - To paraphrase/quote Sokka, the Kataang interaction in the comics gave me serious oogies, because there was almost no tension between them, it was almost all lovey-dovey stuff like they never disagreed on anything - at least that's how it felt to me reading The Promise.
That said, still looking forward to it, big time.