Thanks for asking this. I don't see any reason why you use the same IdP with two different Client-IDs. I haven't tested this, but it is doesn't work currently, I'd like to add it as a feature.
Your description of the protocol is spot on. OpenPubkey currently only works with ID Tokens.
>Is the JWKS response cached? Can it be pre-seeded and/or replaced with an offline copy?
Currently the JWKS response is not cached, but caching support is a feature we want to add.
What is the interest in a pre-seeded copy? Availability concerns?