Question about Belkin Dock Pro

No, Macbooks with M1 processor only support one monitor. It does not matter which dock is used.

That is possible. I have not yet tested it with a MacBook Air M1, but with various older computers with presumably lower performance than the M1 (e.g. older MacPro, Lenovo Thinkpad, Dell Notebook). You should be careful with the cables, though, because you have to connect a monitor to the USB-C/Thunderbolt. The resolution of the monitors is also important, but up to 4K should be possible.