I've tried to book trains in April from Venice to Zurich on Loco2 and Eurorail, etc and every time it gets to the part to confirm they say not available. Seems like there are plenty and trains, with lots of seats. Can I count on buying tickets there later or when I get to Rome a few days before. What's the deal with Italian trains?
Try the tenitalia site.
There are no direct trains from Venice to Zurich, but several different routes you could take. Which route are you looking at? It may be cheapest to break it up into segments. Min any case, buy on Trenitalia, not "Eurorail".
We took the train from Venice to Zurich a few months back. We bought our tickets in Venice the night before. It was a beautiful trip. We had a change in Milan. The route took us through Lake Como area, Lugano and some fantastic mountain scenery and towns in Switzerland.
Absolutely no need to buy your tickets now. Just buy them when you first get to Venice or Rome. There is probably one direct train from Venice, but most will require a change in Milan. While the Milan station is huge changing there is simple. for Schedules check sbb.ch.