Wanting to make a frosted-glass dry erase whiteboard

Removing paint from glass is pretty much always easy. It's hard to scrape it off if the surface is rough and frosted but then again, glass won't be adversely affected by paint removers. If you were to go with film, most of the adhesives that hold them on are pretty serious - always test first.

For this, I would use lots of thin layers of spray paint. This would make the surface appear smoother since I'm sure there's some texture with the frosting, and the spray paint will stick to the low spots and even up. Make sure to mask it off well!

Glazing manufacturers have spent decades perfecting a spacer system that does not eventually leak and condense. They still haven't completely perfected it, though they are much better than 20 years ago. There is no way you're going to put together a system that does not eventually leak, especially with a vacuum. You'll have a better chance with inert gas, but I'm not convinced even professional argon filled glazing stays argon filled years later. It's not that I doubt your fabrication skills, it's just that it's a very difficult problem to address.

For all the time and effort spent assembling a window system, it's worth buying professionally built glazing units. If you want to save some money, forgo the argon filled and simply get dual glazed units with a low-E coating. IMO, these offer decent thermal performance and good value.