There are 16 thrusters on the service module and they only need like 4. One is malfunction. They're trying to diagnose the problem to fix it for next time since the service module burns up on reentry.

Of course they're not stranded. There are other capsules we can send up to bring them back down, regardless of Starliner's status. I'd bet SpaceX is preparing a Crew Dragon in the background, regardless whether NASA has asked them to or not. A rescue Dragon ready to go to save Boeing astronauts would be a massive PR win.

This helium leak that wasn't an issue on the ground is becoming more and more hilarious to me (because no lives are at risk, just Boeing's already shit reputation now). A small leak on the ground, no big deal. Even with the leak, it will last like 40+ days. Oh, now there's 2 leaks... oh now it is 5 "small" leaks. As the days count down and it sits up there attached to the ISS.

No one is saying it, but I honestly think they're worried about whether they can even undock and maneuver the capsule away from the ISS safely and reliably. NASA for sure has teams working on all sorts of contingency plans. if it can't maneuver on its own, either dealing with that airlock being useless for the rest of the Station's life, and a dead capsule that cannot serve as a lifeboat being stuck there; or if there is a way to get the Canadarm to grab the capsule and chuck it back towards the planet out of the ISS sphere of influence. As hilarious as that would be to see, this super slow motion robotic arm grabbing the capsule and yeeting it back towards Earth. Unlikely scenario, but that's the type of thing NASA does all the time, plan for as many scenarios as possible so there's always another option available even if they seem ridiculous.