1. Typically up to 11 months. You've just missed this year's summer sale, but there's almost certain to be another in November.
2. The clock starts on activation, which you can do at any time but most people only activate on the day they board their first train, just to be safe.
3. It varies by train and by country. Eurostar has one of the longest booking windows, whereas domestic trains in some more easterly countries only open for booking with a month to go.
4. I can see why you ask this, coming from the US, but in Europe there are only a tiny handful of instances where night trains are the only option between two places. Most routes of any significance are likely to have a variety of departures throughout a given day.
5. Bags aren't checked before boarding, although a couple of low-cost or airline-influenced rail companies have recently started to introduce some basic regulations around luggage, which are still very generous and not meaningfully enforced. The rule of thumb is that you can take what you can comfortably carry and preferably something you can stow (weight-wise as well as dimensions-wise) into an overhead bin. There usually will not be any help from train staff with luggage. Bear in mind that smaller local trains usually won't have the auxiliary luggage spaces (stacked racks near the doors) that long-distance trains do.
Hope you don't mind my saying but putting the Greek islands between Venice and Rome rather than after both could result in quite a lot of retracing your steps, as apart from Corfu and Kefalonia they are all off the other side of the Greek mainland, which is where the ferries from Italy come in.
It's easy to get from Venice to Rome in a few hours by high-speed train, and also to go on from Rome to a port such as Bari afterwards.
