That struck me too. You don't need to lobotomize the model that answers questions, you just need to filter out "bad" questions and reply "I'm sorry Dave, I'm afraid I can't do that".

Would it be 2x cost? Surely the gatekeeper model can be a fair bit simpler and just has to spit out a float between 0 and 1.

(caveat: this is so not my area).