Is it possible to mount a dataset inside an Instance VM (Incus based) on 25.04?
Not at the moment (easiest workaround is to use NFS), but let me plug my feature request. 
Create Incus VM host path mounts in Instances UI
Can I mount NFS from a VM to the TrueNAS host? I remember the days of VM to host connectivity issues.
Sure, it works fine so long as you follow the instructions to set up a bridge between host and VM: Accessing NAS from VMs and Containers | TrueNAS Documentation Hub