dragon,
As you asked for the "easiest/least hassle" routing, my first suggestion is to change the order slightly. The most efficient route would be Venice > Florence > Cinque Terre > Rome. Are you using the Italy guidebook to plan your trip? When you arrive in Venice, use the Alilaguna Water Bus for transport from the hotel to Venice.
Regarding your questions.....
- There may be a way to find out what direction the seat is facing? I've never been concerned about that. The direction of the seats will only be a concern when using the "fast" trains which require seat reservations, as with Regionale trains you can sit where ever there's an open seat.
(b.) I'll comment further on trains below. YES, 40 minutes is definitely enough time!
(c.) More comments on that below.
- It won't make any difference whether hauling a carry-on and 22" bag on "fast" trains or Regionale trains. You'll still have to haul it on and off the train which shouldn't be difficult with bags of that size. You should be able to keep the 22" bag with you on the rack over the seat and definitely keep the carry-on with you. I would definitely suggest spending a few days in Monterosso, as you won't be able to see much or enjoy the area as much with a day trip.
A few thoughts on the train journeys you'll be taking....
"I read that I won't need to validate the ticket for the Regional train since the PNR code printed on the confirmation will suffice."
That's not entirely true. If you're pre-purchasing Regionale tickets (which is not really be best idea), these will come pre-validated but will come with restrictions. If you purchase Regionale tickets locally, either from a Kiosk or staffed ticket office, you MUST validate them prior to boarding the train on the day of travel, or risk hefty fines!
Assuming you decide to follow the route that I outlined above, these are the trains
- Venice to Florence - you can use either Freccia or Italo trains on that route, since they both use the same stations. Your destination in Florence will be Firenze SM Novella.
- Florence to Monterosso via Pisa - I'm assuming you want to go via Pisa in order to stop and see the Tower? In order to avoid having to store luggage, I'd probably take a few hours one day and just see Pisa as a day trip. You mentioned a day trip to Siena, and that's very easily done by Bus (about an hour each way). For that trip, using trains that only have one change will be a bit easier. You could consider a departure from Florence at 10:28, arriving Monterosso at 12:52 (time 2H:24M, 1 change at Pisa Centrale, 1 segment via Regionale, 1 via InterCity).
- Monterosso to Rome - I'd suggest using one of the direct trains from La Spezia Centrale to Roma Termini. There are several of those each day with a travel time of about 3H:45M. One of the quickest is a departure from Monterosso at 12:42, arriving Roma Termini at 16:32 (time 3H:50M, 1 change at La Spezia).
- TICKETS - tickets for the "fast" trains are offered for sale up to 120 days in advance, and considerable savings are possible with advance purchase tickets. Note that the cheapest Super Economy tickets are non-refundable and non-changeable. Also, tickets for these trains are specific to train, date and departure time, so can ONLY be used on the one train listed on the ticket. Boarding the "wrong" train will likely result in hefty fines! There's no advantage to buying Regionale tickets in advance, as there are no price breaks and these come with restrictions. It's just as easy to buy those when in Italy, either from a Kiosk or staffed ticket office. BE SURE to validate prior to boarding the train on the day of travel. One of the easiest sites to purchase tickets for either Trenitalia or Italo is www.capitainetrain.com (very user friendly website).
Buon Viaggio!