So just to remind myself after the holiday break, you have a armhf board, and you are trying to build snaps on it that have base: core18, and:
1 You aren’t able to use lxd because inside the lxd container you can’t install snaps
2 You aren’t able to use multipass because you weren’t able to get the right config options to enable KVM
Is that accurate? I think that the second option should really be discussed more over at Config requirements and possibly an issue about the kernel config requirements that multipass has (perhaps a new issue at github.com/CanonicalLtd/multipass/issues is warranted to clarify this)
The first option still seems like the path of least resistance for you, when you try to install a snap inside the lxd container, do you see any denials or other suspicious messages in your system log either inside the container or on the host?