I imagine trusting the plugins because they will be tested and source reviewed and realtime checksummed against tampering and peers will be able to validate that it actually ran and for mission critical plugins, we probably need an external verification, eg. blockchain.info for bitcoind operations
This is just wrong - it's like your trusting that other peers are running the correct NRS - of course they may not be and you have no way to know. The only thing you can do is verify results and compare them to what you know (and blacklist peers that disagree with you).
So your verification idea here is "ass about" (it would only be useful for a server to use to check that they are running the correct plugin themselves which would really just be like a "version check" - it doesn't help the "script" itself in any way at all).
Understand that any "step in the script" to "verify a plugin" can *simply be ignored* by Evil Bob's script processor!