Whatever it is, that's a pretty premium style of metal clarinet. Adjustable barrels are hard to find on all but the highest quality metals.

Detachable bell is also a good sign - for some reason, the more pieces a metal breaks down into, the better it tends to be. Don't ask me why.

I would be hesitant about that, as not all barrels are interchangeable - just like how wood or other barrels are not always even going to physically fit.

While most Vito bass clarinets are strictly meant to operate in one piece, there are some that actually do come apart in the center.

They are not meant to, but with enough force you could separate the two pieces. Not sure why you would, but you totally could.

Another (very expensive) option is Music Medic - they can turn just about any metal object brand new, with lots of time and money invested.

That clarinet certainly went a long way from home

Again, I don't think it what you would call a forgery or a counterfeit. There's absolutely no reason to counterfeit something like this. That being said, counterfeit clarinets are almost entirely of Buffet models, with a minority of Selmers.

Could be a replacement bell, for a variety of reasons. Doesn't make it counterfeit or a copy.

As far as synthetic pads, you can find them all over the place, including the ones I mentioned above. Valentino brand are probably the best of them all.

I see your concern with the thumb rest - tone holes on metals are often cast into the body itself, thus tough, if not impossible to unbraze. There's always the possibility of this clarinet having brazed-on tone holes, but I can't tell without it in-hand.

Yes, it will leave a blemish, but seems like a small price to pay if you intend on using this instrument with any level of comfort.

As for the tuning mechanism - beyond me. Acetone dissolves organic gunk, penetrating oil lubricates; longer sounds better.

I haven't heard or seen of red rot being any kind of problem for metal clarinets.