Yes!
I disagree. Perhaps you could link to the relevant terms and conditions which categorically state a weekend first ticket is only valid on one journey with one ticket? That would probably clear the matter up.
I have no idea what FGW have to do with this! But since you bring them up they have this to say about your third point above:
Source:
https://www.firstgreatwestern.co.uk/your-journey/on-board/first-class
So contrary to what you say it appears you can purchase a weekend first ticket, change trains onto a service operated by the same TOC and continue to use the first class facility.
Again this makes no sense whatsoever. How can one journey as defined in the contract (the NRCoC) be contractually separate journeys??? It's a contradiction in terms.
I believe Hadders has already done this. The relevant quote you request can be found in post 12.