Put the seq in slave mode and tick the USB port in midi clock destination in project synchronization setup. This will send a clock signal to the seq and start /stop messages. I suppose, but no seq hardware to test atm.
Yes, that is how synchronization works with Cubase. Always synchronize with Cubase as MIDI master, it won´t work the other way around.
If you want to use the SEQ as a MIDI interface to access your legacy MIDI hardware synths from Cubase, you can configure the MBSEQ MIDI router to forward any MIDI packets received on the virtual USB MIDI ports to your SEQ MIDI OUT1-4 ports.