Hello,
A year or so ago I restored a really lovely Silver King for a client, selling for a whopping $1,300. Quality metal clarinets are quite popular, even the 100-year old ones. Some of them are truly nice, and the Buescher is a solid one.
Response to your questions:
1. Leather pads. Why do we use leather pads on any clarinet to begin with? In my opinion, there are a couple major benefits:
a) rot and insect resistance. Ever notice that just about every old clarinet with bladder pads is completely infested with pad mites? They love the felt material, and can easily chew through the outer skin layer to reach the felt. Leather is much more resistant to these types of insects.
b) conformance to tone hole defects. Leather pads are often somewhat more soft and pliable than skin pads, and can more easily conform to small chips or other defects in tone holes.
To be fair, all of the above also applies to synthetic pads, which overall is probably the best option for any modern clarinet restoration.
You can get them super expensive from musicmedic.com, or cheaper from China, or medium-price from Instrumentclinic.com
2. Thumb rest. I don't believe that your clarinet has an adjustable thumbrest; it is likely welded onto the body and unable to be moved in any easy fashion.
You can try the Ridenour Thumb Saddle, or a similar style from Kenny Woodwinds.
As for the silver plating - wearing off of the silver plate is an occupational hazard. Unless it's never been touched since manufacturing, some amount of the plating WILL be worn off. Anything over 80% plating for one of these older models is a feat in and of itself. So, I wouldn't worry about it.
I can't imagine anybody managing to damage a tone hole while working on the thumbrest unless they're amateur or just being careless.
3. Tuning mechanism. I assume you mean there is an adjustable barrel/neck? A month seems like a long time. I don't see why an overnight soak wouldn't loosen it right up.
4. Case latch. It's highly unlikely you can find this specific latch. Unless this is a museum piece that demands pure originality, any tech can replace both latches with a modern equivalent.
If you wanted the same one, you would likely have to cannibalize a contemporary case. You can find garbage metal clarinets for like $50, and just grab the latches off that case.
5. Silver finish. If you want to spend a couple G's, MusicMedic can replate entire saxophones, so I imagine they can do the same for clarinets.
The Silver King I worked on was pretty solid with about 95% coverage, so I just did a hand-polish and called it a day.
6. Red rot. Not sure what you refer to, but I will infer that this is a rust/oxidation issue? You might see a few rust spots, but typically this is not a structural issue. The only part of a clarinet that really rusts to the point of damage are the screws and rods. I once had a metal that looked like it'd been in the bottom of the ocean, but the only structural problem was the hardware.