You are welcome.
The OEM key works for a "physical" install instance, not the virtual instance. That's why it will not work in the VM on the same device.
Q3: If motherboard dies and you replace it, OEM key is gone. If you have a retail key you purchase, you can absolutely transfer it.
Q4: Unfortunately it is not possible to downgrade from Pro to Home. Upgrading path is supported, but not downgrading path. In that scenario, a clean install is a must.