Only one Priscus voting.

If During any referendum, if there is only one priscus on the table, is he forced to vote for or against?

Do the cases of abstention of the priscus occur only when there is a tie in the ballots sub-referendum?

"The Prisci Block: The prisci as a group have 3 votes. During a referendum, the prisci block of 3 votes is cast “for” or “against” the referendum according to the prisci sub-referendum. Each ready priscus provides 1 ballot for this sub-referendum, and votes may not be used in this sub-referendum. Each ballot cast is either “for” or “against” the main referendum. Whichever side has the greater number of ballots in the sub-referendum gains 3 votes in the main referendum. If the sub-referendum is tied, the prisci block abstains from the main referendum. As vampires cast their ballots in the sub-referendum, the prisci block of 3 votes might shift between “for”, “against”, and “abstain” (as the majority in the subreferendum changes). A vampire must cast all of their votes and ballots in agreement."

I'd answer: It's not, as according to the rules, "there is no obligation to cast votes or ballots":

2. Polling: Cards that are usable “during the polling step” but “before votes and ballots are cast” are used at this time. Then, all Methuselahs may now cast any votes and ballots they have (see below) at this time, in any order. They call out their votes and ballots freely, and there is no obligation to cast votes or ballots.