THe William infidelity thing is based on the idea of William in old WiaB pre rework, when it was written with harem style logic. Current Wiab, we only know that William has a high sex drive, but gets worn out by Miru. William has only flirted with others, and not even much. The other reason for William's suspected infidelity is the theories about the SG cast's parents, none of which is fact. We know none of the fathers other than Nick Benz or Stefan, and we see a few mothers (Amber, Elena is mentione, Layla's mum is mentioned, Ayua is Zane). There is very little confirmed parentage other than legal names.
Most likely Willi will be a massive manwhore though. That'll be his excuse to find out info, just like Leia wanted him to do with Monica.
Other than that, yes I agree, Willi will do the thing with Daphne, it's too big a set up for it not to happen.
Well, there will be a lot of personal canon things happening in WIAB outside Ocean´s official canon, depending on how we play WIAB, e.g. who Willi ends up with, stays with Miru, reconciliation with Leia and Monica, etc.
I think this is the reason there are so many possible kids of William/Miru/Leia/Kati/Daphne et al hopping around in SG, so that the WIAB players can see their personal canon in SG (in a fashion at least).
I theorise the following to be the most likely variant who is who´s kids/parents, going by some developments and things we already know: Willi and Miru are the parents of Ayua, that is pretty much a given. Maybe Ayua has some (half)brothers, since Miru stated in WiaB she would like 4 kids. She has officially female cousins, since it was mentioned in the beer bar that Nika would have to marry Ayua or one of her cousins to learn some special martial arts moves.
Of those cousins, I am convinced at least one is actually Ayua´s halfsister. I consider it 100% sure that when Willi and Leia reconcile, he puts one kid minimum into her and with the consent of Miru, since Willi and Leia can never be together officially. Miru is not only observant, Willi also promised her that one day he would tell her about his love from before. I estimate Miru already has some inkling that was Leia, too much hints that way even for an outsider.
If Willi and Monica reconcile (it seems possible in WiaB) I could see one of her kids being from him. I also agree with others that a Daphne-Willi tryst is nearly a given, but it does not have to be an affair. Willi could be only a spem donor for Daphne, that is one thing I think Miru would be open to. It would also fit into the thing why Nika and Nami ended up where they are. Miru might be ok with a sperm donor thing, but not with having Daphne (and kids) around all the time. (I find it likely that Daphne is Nami/Nika´s mom, esp. Nami looks very much like her daughter)
Kati, Dana and Zoey are also possibilities for Willi to have kids with, but further out ones. I consider them possibilites for personal canon stuff, not official canon ones, with the exception of maybe Zoey.
Miru is the mom of Ayua and maybe some sons, since she wanted 4 kids. Beside Willi, I think the only other possible fathers who might make it into official canon would be Dylan and one of the porn actors.