Thanks, I was able to reproduce your issue.
Unfortunately adding arm64 stage-packages on an amd64 host is not trivial, unlike adding i386 packages on amd64 with dpkg --add-architecture i386. You may have some luck referencing the apt commands run in GitHub - canonical/cross-build-utils or here.
…And guessing how confined the snaps want to be, it’s likely because the multipass is limited to
$HOME-folder?
Yes, Multipass only has access to your home directory by design. You can access /mnt and /media by connecting the interface.