Lenovo might have intentionally reduced the performance in Windows by default to enhance battery life, because it is a gaming laptop they probably have a lot of performance options in the vendor’s control center to sacrifice battery life for performance, are you sure you activated every performance option?

Also mesa drivers often have better performance than Radeon official drivers, but they are less consistent (more stuttering).

Probably, if everything else is correct. GPU passthrough hands over an entire GPU to the virtual machine, so the only way of getting the output of that vm is plugging a monitor in that GPU

I haven’t looked into how to configure this but it should be possible, and you would use the motherboard HDMI port for the VM, and the ports on the dGPU for the host. As usual, the arch wiki is your friend, even if you are not using arch

But… If you don’t care about VM performance (seeing as you are passing the iGPU to it) you should look into other options like virtio or sr-iov, so you don’t need to fiddle with the HDMI ports. Please notice that virtio is paravirtualized and only works well for Linux guests, and sr-iov is real hardware virtualization and requires hardware support. Both these methods require only one GPU. Once again, look at the arch wiki and the qemu wiki.

Also, if you are using Linux guests, you should really look into “GPU native context” which is a paravirtualization method that works similarly to Hyper-V’s GPU paravirtualization (which is currently the best) and would allow almost native performance for the VM, without requiring multiple GPUs. It is not available for amdgpu yet, but you can follow development here.

P.s. if you are using windows hosts, paravirtualization methods will not be satisfactory for the foreseeable future. You will need either passthrough (like you suggested) or full virtualization (with sr-iov). I can give you more details if you like.