I’m not much hoepful, but… just in case :)
I would like to be able to start a second session in a window of my current one (I mean a second session where I log in as a different user, similar to what happens with the various ctrl+alt+Fx, but starting a graphical session rather than a console one).
Do you know of some software that lets me do it?
Can I somehow run a KVM using my host disk as a the disk for the guest VM (and without breaking stuff)?
i do not think it is possible, but perhaps the closest thing u can get is virtualbox or qemu/kvm/virtmanager with shared folders, i believe that allows you to passthrough files between guest and host. (shared folders allow you to copy paste something on ur host into them and have them appear in the guest, but dont think i used them before, at least not extensively, so you might have to look into them to check if this advice helps you)