Well, guys, maybe you're not familiar with beta or nightly builds.
There is no point installing them and using them unless you're willing to experiment with whatever's implemented (detailed in the release notes) and ready to face whatever bugs you will experience.
New features needs testing before being implemented in a future stable release.
All your complaints are simply related to the fact you are using an unstable preview release of a browser and therefor is irrelevant.
1. & 2. the preview uses a different profile, that's protecting your primary and default one.
3. Since it is a different profile, of course it doesn't sync with the default.
4. Installing a beta system is an all different concept from installing a browser beta, a frontend developer (for example) might want to experiment with a beta browser without having to install and deal with an unstable system, that's the point.
Seems like you have a problem with Apple that goes way beyond a browser preview release, an all different subject.
About your BTW paragraph, same as above, different profile is way better than using your default one, guess you would have been complaining if it did mess something up in it anyway.