You shouldn't need to sneak your bikes from the train in to Venice. You cannot ride but walking a bike seemed okay. We were in Venice in May 2014 and saw a few people walking bikes. You will probably have to deal with a bridge or two. Some of the smaller bridges consist of stairs (up & down). We happened on a group of four touring cyclists (Germans, I think) and I helped one of the women get her loaded bike up and over two of the bridges.
Enjoy your trip, we did a similar trip in 2014 but we took the ferry from Venice to Croatia.

Tailwinds,
Charlie