I'm not sure Ocean knows how it will work either (see below). But today it seems we will have some emergency Kat instead.
WiaB is planned to be shorter than SG, but given current trends, we'll be probably into SG season 4 before WiaB starts winding up. Complete guesstimate, based on WiaB having fewer LI's, and fewer chapters planned (less branching, plus shorter). Even then, I can't imagine anyone who has already releaded season 1 and played from the start to fix a mistake they made early on is unlikely to reimport their WiaB save and replay to notice all the flavour text.
I think it'll be flavour only, as I vaguely remember Ocean saying something about wanting to avoid a canon ending for the romances, to keep WiaB from feeling like there's no risk. Also to avoid players getting upset that their choices didn't matter in the end regarding their LI.
I imagine thinks like backshots comment about William will change depending on what you actually did, but it'll be (I think) only flavour, otherwise Ocean needs to add more renders to accomodate the changes. I think the same thing will happen with SG to some extent, as far as the main story goes. I'm guessing that even if Nika is unhealthy, he'll still get his mojo back for the basketball, but there will be dialogue suggesting he's not pulling his weight. He'll still be on the team and all that. I haven't tried it, but even if you lose the tennis game, I guess you win the street basketball? That's kinda what I mean - Ocean's not going to bury himself under implementing every possible choice, like Nika staying a noodle. Too many renders then, it'd have to be rendered on the fly to accomodate that level of change.
So the WiaB flavours will probably mainly include making Ayua awkward with all the stories about William, one liners from milfs who knew William and want to show their superiority by claiming one night stands or some such.