I think the issue with mount is not a restic issue, but a permission issue of the user who runs restic mount. If I mount with user root, I see the right users and groups.
If you want to just restore some single files, mount (using user root) can be an adequate solution. Keep in mind that access to files in the mounted dir actually means that the content data must be downloaded from the repo which can be expensive e.g for remote repositories. This also holds when using rsync - even when parts of the file in the repo and in the destination match, all data must be read by rsync to find out what actually matches. With single files I guess it’s ok - unless your single files are huge, like e.g. disc image files.
As an alternative you can try rustic restore (see https://forum.restic.net/t/a-restic-client-written-in-rust) which allows to restore single files or dirs, supports restoring to existing content (using what’s there to only restore needed file parts) and has a --delete and --dry-run option.