I caught my regulator before it fried my battery and it would be around 14v at 3k but it would just keep climbing up to atleast 19v but it was worse when warm. Honestly I didn't have any running problems but that could be due to my new (then) Odyssey.
I know you said you visually inspected all connections but in this case I would really focus on the fuel pump connections. I.e. Take apart connections and clean with contact cleaner. I've found mine with some corrosion before.
You seem like you know what you're doing, we can get this!
Fwiw this seems to have worked on my r5 atleast for now...
On the fork seals?
Yes sorry, 46 year old fork seals it seems to be holding otherwise I'm out of fork oil....
Since acquiring my 71' Yamaha R5 I've been posting on the " 2 Stroke World" forum. Although very informative and active, no way is it as good as this forum.