IIRC the Crew 9 mission was postponed for exactly this reason. At some point, Starliner needs to be kicked out because they need the docking space. They can't keep postponing ISS missions as they please.
Exactly right. If a bunch of thrusters don't fire up again you now have a huge piece of debris at risk of colliding with the ISS. This probably gives NASA the most pause before doing an unmanned Starliner exit. Having people on board Starliner might be able to recover from more thruster problems but then there'd also be people on a death trap. So an unmanned Starliner might be risking as much life as a manned Starliner and NASA has no idea how much.
As for your earlier question: yes. Starliner has to leave before Crew 9 can dock. And their rules are it won't launch Crew 9 until there is a port for it to dock to.
On the ISS there are 4 ports on the Russian side only compatible with the Soyuz / Progress ships and 4 for the US side. 2 are "Common Berthing Mechanism" (CBD) used by Cygnus cargo modules (and the original Dragon 1) and 2 are "International Docking System Standard" (IDSS) used by newer Dragon 2, Starliner, "and future" vehicles.
The result is that before a second Dragon can launch Starliner must leave. If Butch and Suni aren't on it then Crew 9 arrives with 2 empty seats and 2 new space suits. The contingency exit plan in between Starliner leaving and Crew 9 arriving is for Butch and Suni to lay on the floor of Crew 8 Dragon without pressure suits below the 4 Dragon crew members (their Starliner suits can't plug into Dragon's systems).