Venice: Can we get from A to B to C in 2 hours? (details below)
 

1. We arrive at Venice train station at 6.30pm (late April)

2. Need to catch transport to our hotel, then check-in, and eat something

3. Would love to attend an 8.30pm performance at Musica Palazzo

Q: Would we have time to a) walk there from our hotel (Hotel Galleria) or arrive by Vaporetto (the #1 seems to be convenient) ?

Map here shows locations of hotel (near Accademia Bridge, centre left) and Musica Palazzo: http://www.musicapalazzo.com/contatti/

It will be tight, but yes.
It is the meal that is the question. The Pizzaria Foscarini is right at your hotel---try to order as you arrive.

I'd eat something on the train and then dine after the concert.

I doubt you'll find a restaurant open and still serving after the concert that time of year. Best to get a quick pizza or panini on the way to the concert.

You won;t have time for dinner - but if the train is on time you probably can grab a snack on the run.

Walk there. You will be very close. I would say if you have time, go to da Fiore on Calle Botteghe just off Campo Santo Stefano and stand at the bar for cichetti and wine. From there it is a 5 minute walk to the music venue.
http://www.dafiore.it

If you can't eat until after the show and are starving, there is a place in Campo Sant' Angelo called "Galileo" that may still be serving. Or take the vaporetto up to San Toma and go to Muro - they serve late. Have the hotel double check with them when you get there, their closing time that night.
http://www.murovenezia.com/