What protocols are the zones you want to group? Airplay , Roon Ready, Chromecast etc You can only group zones that use the same streaming protocol as they can’t all.play in sync as they all have different streaming tech that does not play well together . So Roon Ready devices can only be grouped with other Roon Ready same for airplay with only airplay so.on and so forth.
(The truth is out there but not necessarily here)
Using one license on two cores in separate houses works OK. Cannot use simultaneously (although I got back to one house and it was still playing even while I was using it at the other house, but could not access the interface - it was just playback continuing while Roon displayed the “unauthorize core” window). To sync the libraries you have to backup the database at one location and then restore it at the other. That screws up the audio endpoints and then they have to be reconfigured.
Using 2 licenses doesn’t allow the backup-restore and so there is no way to sync libraries across cores with separate licenses, as I understand it. I’ve never tried since I only have one license, but I read that here somewhere.
No, as mentioned in the KB article linked to above, only Zones of the same type can be grouped. So Sonos Zones can be grouped with other Sonos Zones, but Sonows Zones cannot be grouped with any other type (like the Marantz).