Agreed. If you're not willing to say "Nope, you crossed the line. See you next time, I'll decide when that is, goodbye" (or similar) and leave (cut them off to whatever degree is needed to stop the harmful behavior), then you need to not give them an opportunity to start again.

Yes, that's important. You actually have assert boundaries and simply communicating them might not be enough. At the same time it's very valuable to go through the experience of asserting those boundaries. Parental relationships do have a strong effect on the human psychological system.