Can they transporter a tractor trailer & what would it cost?

Yes they can. They charge by the foot in that case. I see 53'ers all the time on the ferry.

Yes, commercial rigs cross back and forth throughout the day, both directions. Oversize and overhigh rigs must be scheduled in advance to take advantage of off-peak crossings. See http://www.wsdot.wa.gov/ferries/fares or search for Washington State Ferries fares.

I dont think so. Youd have to take Deception Pass