Could someone accidentally swallow a "boil & fit" teeth grinding mouth guard?

I doubt it. If you are talking about the ones that cover your entire upper arch, then that's pretty impossible. However, there are some versions out there that are very small and only cover a few teeth. That, while remote, could be a possibility...But still pretty uncommon.
No. The whole-arch boil & fit appliance is to large to be swallowed. It's also nowhere near as effective as a custom made appliance. Ask your Dentist for customized personalized help. This isn't a DIY get what you pay for.
Unlikely. They are too big to swallow. But they should fit well and not come loose. Maybe your dentist can evaluate the fit.
I do not believe so. Anything that covers all of your teeth would be too big to swallow.