@robbertkrebbers suggested fixing #274 (closed) by making IntoEmpValid instances Transparent.
It turns out also one redex of tac_pose_proof must be exposed to the guardedness checker; making such a lemma transparent seems a bit ugly. I wonder if this application should be built in Ltac instead, but that's up to @robbertkrebbers.
Nevertheless, this fix appears technically correct, and has no excess transparency.
I also made proofs of IntoEmpValid instances more manual, even tho that's not strictly required, because they're Defined and their exact body matters.
Moreover, I modified the testcase in #274 (closed) to use all instances of IntoEmpValid.