I already raised it as bug 109296 some time ago, the response was that it couldn’t be solved using the snap interfaces available, which AFAIK is indeed the case.
Specifically the default confinement doesn’t allow LO to access anything under
/run/user/<uid>/gvfs/..... That’s why I believe that it would be appropriate for LO to use classic confinement at least until suitable interfaces become available.
Re opening documents owned by someone else: it can be indeed by browsing to another user’s directory, or by having a common directory with documents accessible to multiple people, or, in my case, it happens when I use an unprivileged LXC container to run a script that generates a DOCX file in my home, which is bind-mounted in the container.
Generally speaking: I believe that the classic confinement is adequate for apps that are basically little more than internet clients, but something like LO is and should be expected to be able to work with the full range of facilities offered by the OS.