No, a Bernina dealer won't "void the warranty." We get machines in that DH has tried to fix and just made it worse, quite often. We still fix them. You can't go to "Bernina University," unless you're a Bernina dealer or tech. But, you don't learn much from PowerPoint presentations, which is what most of it is. Ideally, you could become apprenticed to an OSMG. I've been doing it for 5 1/2 years, and still learn something new almost every day. The new computer machines are a different challenge than the vintage mechanical machines.
"Bobbin Snot," is a great term! Yes, that's the main thing that needs taken care of in the shop. If someone brings in a plastic Singer, we show them how to clean out their bobbin area, and send them on their way. If we find a simple thing like the needle in backwards, we send it back out without a charge. It's hard to charge someone $70 to clean out their bobbin area. So, you try to educate the customer. But there are some that won't learn -- they'd rather pay you to get your hands dirty.
I'd say about 75% need needle plates and hooks polished. And maybe about 30% have timing issues.
There are so many things that aren't in the service manuals. You've got to have a mind that can figure things out. Or an OSMG on the bench next to you.
Sewing machine repair in a shop is pretty fast paced. There's no time for lookey-loos.
I do go to quilt guilds and teach a hands-on sewing machine maintenance class. It's a lot of fun!