In United States tort law, there are two big things that matter: Harm and wholeness. Harm is the thing that actually causes/caused damage to the suing party. Wholeness is the status of the damaged party before the harm occurred. In a civil lawsuit, the goal of the party suing is to prove harm occurred and get a ruling that requires the defendant to make the suing party “whole” again (or as close as can be reasonably established). You’re correct to say that anyone can sue, but my point is that nobody would have the grounds to unless the Texans/NFL didn’t even try to fix things before the lawsuits started. Let’s go through the list:

Stadium

Unless the NFL tries to force the stadium to pay for refunded tickets to fans, the stadium has no damages because it still made the money it would have otherwise.

Seahawks

Realistically speaking, it’s going to be difficult for the Seahawks to claim they’ve been damaged by an automatic win. Again, unless the NFL tries to force the Seahawks organization to pay for refunded tickets, it’s going to be very difficult for the Seahawks to claim they’ve suffered legitimate harm.

Advertisers

This is, again, something the NFL would likely take care of (or, more likely, force the Texans to take care of it) by refunding commercial costs. Again, no damages.

Fans

Fans would have agreed to certain terms, which would include not suing the NFL over something like this. They’ll get tickets refunded and, if they paid for a hotel or plane tickets, tough shit.

There's just no situation where the NFL even finds itself needing to settle a lawsuit, because nothing would get that far. This is especially true because any entity suing the NFL is essentially severing its relationship with the most powerful organization in television, which is fucking stupid.