Clutch not fully disengaging: Low on fluid, flex/soft hose, leaky master cylinder, obstructed mechanism are likely culprits. You said a mechanic checked this all as good? Hmmm. Obviously, it's disengaging, just not 100% and it's creeping when stopped. What is "good pressure"? Is it pushing the right distance? Could you have good pressure in cylinder and have a soft brake line? It sounds like it's only slightly out of range.
You're saying that if the factory installed the clutch backwards it would display this issue ten years later? Wow, dunno.
I'd concentrate on the hydraulic system first.
(BTW- if you post this thread on Wild Guzzi and you can bet someone will offer you $500 to take this terrible bike off your hands)