Nothing about that looks acceptable. I would never, never butt join pieces where any type of load is expected and would never accept replacing requested material with " ordered the wrong" material. Especially with a different size.
And the tack welds look hideous. If it's not TIG it's wrong...
Depends on age (yours) and financial ability.
Is this "the last rig build" you'll do? (Age question)
Can you afford to start from scratch? (Financial question)
My $.02 is sell and get what you want/need to travel as you wish.
The FRP directly over the exterior should work fine without any real extra effort.
I think as long as it's sealed at all corners and seams it would be waterproof and hold up well. What method of adhering are you considering, screws, bonding adhesive.......... ?
FRP/Filon board is absolutely the best way to proceed. Way less labor intensive and more cost effective.