I can't help you with the other questions but looking at www.eurail.com the Italy Saver Pass is EUR 126 per person for three days of travel.

Single tickets (2nd class) on the fastest trains (Eurostar) for your itinerary come to a total of just under 140 EUR per person.

In my opinion, if you can't find out if the Eurail Pass is eligible for Eurostar trains and taking into account that even if it is you would probably still have to go to the train station to make a reservation (reservations are compulsory on Eurostar; don't know if they can be done online), I think it's just more convenient to buy the tickets separately.