I see what you are trying to do as Thurles - Sallins - Heuston is cheaper as two tickets
The add on 'extension' will be what you paid for the ticket you hold against the full ticket price so same as Thurles Dublin. No staffed office in Sallins so can't get an extension ticket there.
Won't make any difference, Thurles Dublin return as one ticket will be the same price and avoids any potential hassle with extension tickets and so on
Note the break of journey is valid same day only