On the specific issue I raised, the BIP only says "Querying multiple

nodes and combining their answers can be a partial solution to this"

which is not very helpful advice. That's a partial answer to my
question #2 with zero response for question #3.

I'm sorry you think it's unhelpful. It is nonetheless the best that can be done within the constraints of the current Bitcoin protocol.

This sort of thing really needs a warning label like "use only if you
don't have a trusted solution" and discussion of that choice is
completely absent (question #1).

It's absent for the same reason it's absent for all the other protocol BIPs: the ability to use a trusted third party is always present and a possible answer for any problem in Bitcoin. So I figured it didn't need stating.

How about adding the following sentence:

"If the above constraints are insufficient for your use case, you can alternatively query a block explorer or other trusted third party to obtain the same information".

Would that make the BIP clearer?