Security of OFB mode if PRP is used instead of PRF
I think the final negligible term depends on the way you prove the rOFB security. If you construct OTP in the ideal word based on random functions, then use PRF; if based on random permutations, then use PRP. Note that in the latter case, replacing block ciphers with random permutations cannot guarantee that no collisions happen among the block cipher outputs. In other words, such collisions are not the same as those considered in PRF/PRP.

The attack surface for PRP is not negligibly higher. It seems higher just because you first prove the security with PRF and wants an easy reduction based on the advantage difference between PRP and PRF. Btw, any negligible difference should not matter in the security proofs.