We've (semi)successfully bought tickets and taken trains in Italy before, mostly @ station, shortly before trip, but I still can't claim that we truly understand the system.
Traveling from Venice to Varenna (change in Milan) in April, then back thru Milan ,a couple of days later, to other points after our stay. I can book all the tickets on the Trenitalia site. Deep discount tickets Venice (SL) to Milan is on a Trenitalia "Frecciabianca", which lets you choose seats, even in 2nd class (fairly restricted as far as changes, etc.). Milan to Varenna is on a regionale actually operated by Trenord that doesn't offer reserved seat in either class. Since we're coming back thru Milan, we would need round-trip (Milan-Varenna), or additional return ticket only from Varenna, so would have to do two bookings, anyway.
a) Is there any advantage in buying the regional tickets on the Trenord site rather than Trenitalia? (no price diff.)
b) Do the regionale tickets still follow the usual regionale rules (essentially "open" for same train run until validation, so could take a different time if necessary...if train from Venice were late, or we wanted to alter our return time to Milan), or are they bound to the booked times if bought online? Would this change for 1st class since there are no reserved seats on those runs?
Edit: Looks like Treniatalia will not let me book another carrier's regionale over one week away unless it's booked on an itinerary along with one of their trains. I can book Venice to Veranna, now, but I can't book a separate return from Veranna or a Milan-Veranna round trip separately. Trenord site lets me book for April.