That is correct.
This is the exact opposite.
Yes and no.
Two masters don’t have to be compatible if you don’t want to interpolate (or generate instances). If you want to experiment with different weights between your master using a variable font you are, in fact, interpolating. So either you want to interpolate, or you don’t
Stephen Nixon of Arrowtype has a repo on GitHub aiming at variable font preparation. varfont-prep
With some tweaking you can generate a variable font that only contains compatible glyphs and leaving out those that aren’t. (It’s UFO based though, but glyphs can also export UFO files).
Another workaround would be to set all incompatible glyphs to non-exporting if you want to stay in glyphs entirely.
Long story short:
Non compatible glyphs/masters = no interpolation = no variable font fun in the browser