a) and b) are plausible, I've had the same thought. c) doesn't mean anything in regards to Ayua's parentage, and Ayua mentions only people who marry her can learn, so she's making it more exclusive than Miru, who's willing to teach Leia. In c) we still don't have a reason for Ayua to be Asian, if it's only Willi and Leia. But Ayua invites Nika to train at the official dojo, so it can't be her trainging in secret - Miru would notice the stolen techniques slipping into Ayua's routines.
Option d) William has a side fling (Riri?) that he bought with poundy coupons, just to see a comparison so he knows whether or not Miru is the best of all Asians (yes, Willi in an open relationship is retarded enough to try that). Ayua gets left with Willi and Mirus raises her because Willi's a jerk even if she sticks with him.
Option e) Willi and Miru have a massive falling out because Willi's chasing Helen which is too much even for Miru's high sex drive, so Miru marries the abandoned Dylan. No NTR if she's been ditched. Will still get way too much backlash.
Regarding secret techniques a further option: Ayua is playing with the western racism/fetishism trope assuming that all fit asians have secret fighting techniques, and are deadly weapons, rawr. Secretly, they're just going to box like regular people. But heck, the trope exists purely because all asian comics, tv shows, movies have protagonists with ultra mega secret fisting techniques. Nika better be careful, I don't want to see that lewd ...or maybe that'll be Ayua's gift to Bella, secret technique to get Nika's prostate functioning? Maybe that's why the secret technique isn't in season one? /s
The most obvious option you've put in the spoiler is the likeliest, and the one I prefer
Sasha's path is what I'm waiting to see. Robin, and Maya I'll keep hoping for lewds. I think Ocean might consider Robin as a LI because we've already got tension from Charlie. I can understand Maya being off limits for a full blown relationship, she can't bring Nika home when Vic's there etc.
There are plenty of people IRL that have no happy endings, and there are plenty of people that do make these kind of decisions. But they're not usually successful in the time Nia is. Just consider it - she makes enough money for that place, having built her career in the time that Nika took off from injury/recovery and Nami stayed back. Seems a pretty fast success. So if you want depressing, she's living in a dump, spending all her money trying to get stable internet, and dealing with 100% douches whenever she's online.
I think the important thing with all these characters is Ocean is exploring all the ways that broken people can help each other heal. Some of the dramas are bigger than others. I mean Nika's circle of friends is filled by highly dysfunctional people. Emotionally, it's like they're the X-Men - each has their unique trauma/origin story and each has a +10 roll in some area of life that makes them survive. I hear what you mean, but we people sometimes have the most stupid reasons for ruining our own lives.