Technically yes. Travel must be completed within 4 hours of the first validation. So in the first case, let us say you bought a ticket from Venezia S. Lucia to Monselice and validated your ticket at 8:30 and boarded the 8:42 Regionale Veloce, then got off at Padova. You could then board the 12:10 Regionale Veloce at Padova that arrives at Monselice at 12:29 and you would be legal by 1 minute.
However, this strategy only saves you 2 EUR. The cost of a ticket from Venezia S. Lucia to Monselice is 5.45 EUR. The cost of a ticket from Venezia S. Lucia to Padova is 4.10 EUR and the ticket from Padova to Monselice is 3.35 EUR. Might be easier and less stressful to buy individual tickets.
From the google translate version of the Conditions of Carriage:
- The hourly validity starts from the time the ticket is validated.
Travel must be completed within 4 hours of validation, with certain exceptions laid down by specific
regional rates.
Tickets purchased online are valid for 4 hours from the start for the selected train and not
require validation
Edit - Actually, the next paragraph in the Conditions of Carriage:
intermediate stops - The traveler can make an unlimited number of stops during the
validity of the ticket.
The traveler who is on the train at the expiration of the validity of the ticket may continue their journey
already it started until the destination station without intermediate stops except to change
with the first useful train.
So it appears you could board the continuing train as late as 12:30, but looking at the schedule, the next train is at 12:41, so to be legal, you would have to have validated the original ticket just before boarding.