That means that we can define a subroutine within the paperclipper which is functionally isomorphic to that agent.

Not necessarily. x -> 0 is input-output isomorphic to Goodstein() without being causally isomorphic. There are such things as simplifications.

If the agent-to-be-modelled is experiencing pain and pleasure, then by the defendent's own rejection of the likely existence of p-zombies, so must that subroutine of the paperclipper!

Quite likely. A paperclipper has no reason to avoid sentient predictive routines via a nonperson predicate; that's only an FAI desideratum.