As you said, yes, we do know there is an ending, probably THE original ending also given Fujino's previous works, where Kirito won't know anything, we still don't know HOW the one where he finds out will be handled though: Fujino could go by having him find out without Inoda/Asuna knowing or have them reveal it to him to humiliate him further, or even have them find out he found out without him realizing this last bit and just pretend they don't know why indirectly trolling him XP
I reckon we're going to get scenes where Kirito gets off to watching two people fucking, before he notices it's Asuna. We had one in Part 1 where he peeps through a door in the inn to see two people fooling around, but back then he wasn't into it. But then, maybe if his Prurience is high enough, he'll start fantasising that it
is Asuna. He also has the subplot friendship with the green haired guy (who I think was the green haired priest from Part 1) to sort of induct him into the ways of being a cuck. It could play on Kirito's insecurity about his size, and be framed as wanting to make sure Asuna is satisfied... even if someone else has to take over that part of the relationship for him. By then, I expect Asuna and Inoda will have been fucking madly IRL, but the situation will contrive where Kirito thinks Inoda is the best person to take care of Asuna like that, for him.
Regarding Sugu, while I would love an open ending where she just keeps riding her Onii-chan whenever she wants to, her current 'route' seems to be sadly against that: Her growing Yandere-ness is escalating more and more, as we saw with what she did to him in FOG and even wanting to let Leafa call him "Onii-chan", which all seems to be gearing up for a 'reveal' (Especially given she apparently won't use her pussy until they're both conscious to do it, in her own words). The issue is that, obviously, THAT wouldn't go well for him because unless things dramatically change Kirito would reject her in a much harder way than in Canon, and how she would react to THAT may determine how her story ends.
We're probably not going for a super dark ending for everyone; sure SAO has teary moments but the tone is usually light. The way I interpret her words, her saying 'I want you to be awake the first time I give you my pussy' may not have been a Yandere
threat that she's just going to rape him one day, but more that she still holds out hope that she can win him away from Asuna eventually. That's far more in character for Sugu, and was basically the crux of her plot/growth in the show; that twisted hope that her brothers relationship one day falls apart so she can be there to pick up the pieces.
Also think about the potential future of Sugu's plot. It's in both Liz and Sugu's interests to work on breaking Kirito and Asuna up, so Liz and Leafa becoming temporary alies for that goal makes sense. But to do that, Sugu first needs to be more properly introduced to the gang, rather than being a side character that only Kirito ever meets. It's likely then that Sugu might notice Asuna and Inoda cheating in FOG long before Kirito does, which she might then start using in her own interests, like cockblocking Inoda by directing Kirito to their fun times so they're forced to stop or get caught. She can't just
tell Kirito that Asuna is cheating, she needs him to
see it for himself to believe it. And I'm guessing there's a lot of potential for the Prurience score to come into play here.