So most of your lovely suggestions were already booked for some of our nights, but using booking.com we found a place in our price range with good reviews that looks like it has a fairly straightforward walk from Ca'D'oro stop. Hotel Mignon. It has free cancellation until next week but unless we find an awesome deal before then I think we are satisfied with the choice.
A glance at the reviews shows it is probably a pretty decent place.
Here's a probably dumb question, but assuming vaporettos go in both direction which direction do we want to go from the bus station?
You'd want to go toward the train station (Ferrovia) and Rialto. Look up directions on Google Maps from Piazzale Roma (bus stop). You'd take the #1 vaporetto toward Lido to Ca' d'Oro. Depending on the timing, taking the #2 to Rialto may get you to the hotel faster but will involve more walking. (#2 boat has fewer stops so is faster.) I'd take the #2 only if it gets you in 30 minutes earlier or something; if 5 min earlier, just wait for the #1 for the shorter walk.
If there's any chance at all you'll have a working smart phone, Google Maps can tell you all of this in real time: when the next boat is coming, how to walk where, etc.
And is there any reason to pre-buy tickets online or just get bus and vaporetto tickets at airport.
Might save you a little time in case you are just about to miss the next bus. I've never flown into Treviso. At Marco Polo, last time I did buy my bus tickets to Mestre in advance. Didn't really help me - my ATVO bus was late anyway. Had I not, I might have taken the slower ACTV city bus because it left as soon as I arrived. At Marco Polo, there were lots of people there to help me and direct me, even sell tickets. I have no idea what the bus options are from Treviso, though.
Also, I've been told that the airport busses raison d'etre is to funnel people to Venice, so if our plane is late then the bus will still be there waiting. I hope that's true!
I don't think that's necessarily true. Planes arrive at different times. If there are people on the bus from another flight, is it really going to wait around for another late flight? I wouldn't count on it.