You can already select the page in the hierarchy using the “Navigate schematic hierarchy” top-toolbar icon. Click it, and the Navigator pops up. The lower-level sheets in the hierarchy are all there. This is true, even if you instantiate multiple copies of the same lower-level sheet (think: “Channels”). Each channel sheet is listed in the hierarchy.
Absolutely NOT, unless those signals are explicitly made global. Again, consider “channels,” multiple instances of the same sheet. The nets must all be unique, or else the whole thing fails. And if you don’t use channels, and instead just have several different lower-level sheets, the right way to connect them is with hierarchical labels on the lower-level schematics and hierarchical pins on the respective sheet symbols in the instantiating sheet.
They already exist, and have for some time! I use them in every design. Why do you think this capability doesn’t exist?