He hasn't exactly said there's nothing they can do about her, he said they can't do much. They might be able to manipulate the rules of her game against her, or they might be able to subvert her in some way. That'll probably be explored more towards the finale though because Mars' mission and her conditions are a complete contradiction at the moment.
Nitro said
here there are individual endings, a harem ending and a bad ending. I'm sure we'll get a few more bad endings but it doesn't sound like they're necessarily the norm.
I suspect the individual paths will be a split between good and bad ends. In that comment Nitro said that something might happen that prevents Vi from being with Mars on some of the individual endings. Kinda goes back to the existing bad end here with the backup priestess. On the other hand the Erin solo ending sounds like it could indirectly fulfill the requirements of the trial, and the Vi solo ending could easily just be Vi and Mars leaving the party after the quest is over.
He also said
here that the harem ending is a bit more predictable. Which in the context of how he said it leads me to think it's a little more vanilla, which he also describes the fem Erin content as
here. So if it's going to be a more vanilla-ish harem ending then I suspect it's going to be happy, at least for the party.
So while only Nitro knows I don't think we're getting nothing but bad or bittersweet endings. I think we're going to have multiple mixed endings, where a few characters here and there get a happy ending and the rest get left behind in some way, an ending where everyone gets a happy ending, and at least a few endings where everything goes to absolute shit. If he's said otherwise then it would've been in Discord or Patreon, because he's made no indication otherwise here.