Windows-Whonix-Workstation in KVM

Yes, its just easier to use custom-workstation but has no special thing that makes it better in security or anonymity if you do the same manually.

No, if you know how to make your Custom-VM in KVM and connect it to the GW then thats it.

Read KVM documentation in case you didnt, and import WS,GW to KVM and see how they are configured.