Thank you for providing further informations about Mir. I wasn't aware that Canonical dropped the libhybris integration. And thanks for taking care of that!

Point by point:

It's probably easier for me to explain with an example. The LibreOffice snap in the U.Store uses the "Home" interface, which was meant to be transitional. Now that Canonical has no plan for Ubuntu Personal, could we expect similar interfaces to be "standard"? Could they break (in terms of UX and security) the current UT/UP security model, which relies on ContentHub for content sharing?

Huh, when I used "content" I was referring to files, document, or more generically data. I wasn't aware of such interface.

Yeah, that was my fear. The only example I found for adding new interfaces is this one. My impression is that Snaps have been designed with a strong centralization, afaiu.