Can you book the ferry for one day and come back on the ferry the next day and it is still the round trip price?

Yes, as long as you make a reservation. It will take 4 hours each way by ferry. The trip back is at night with no scenery, no music, hung over / drunk people in bad moods & contagious vomiting. If you take a plane to KW it is only an hour each way which is more enjoyable & gives you much more time for your vacation.

Yes the only change in price is the cost of parking

Yes. We went on Wednesday morning and came back on Thursday night. Make a reservation.

Yes you can

Yes

Yes, and cheaper than flying

Yes and yes

Yes you cam woth some limitations. Check the trip calendar when scheduling for availability.

Pretty sure that's the round trip price

Call

I am not sure.