I can't tell for sure about the three first questions (although I think there's no support for booting something else than the internal Sailfish at the moment, but it was said that efforts are made to open the bootloader), but I suppose any Android application not requiring Andriod >4.4.4 should run.

There may be some functionality limitations here and there depending on DRM requirements (I'm having that issue with Molotov on Jolla C for instance) or unsupported features (I don't know if the Jolla and Youyota tablets have support for ad-hoc WiFi for instance, and some Android applications need to connect to sport cameras for instance). Are there other limitations related to the architecture?

The only thing my brain can read from this message is you have a long term vision about the Sailfish devices that you could produce. And knowing what kind of devices you have in mind for the first steps (trying to revive the Lauta concept and HW keyboard devices despite what the mainstream phones currently are), I can tell that hype is real.