At the moment, it seems difficult for the PD (plus allies) to form a minority government. That’s because abstentions in the Senate count against the passage of a motion, including a motion of confidence in a new government.
If the PDL wants to prevent any PD government from forming (and there are good reasons for thinking it would in fact want to do so), and if the M5S will not vote in favour of any government, then all the PDL has to do in order to defeat a PD government is to copy whatever the M5S do.
If the M5S abstains, then the PDL should also abstain. Their abstentions sum to 117 + 54 = 171, which means that the votes in favour cannot be more than 144, which means the motion does not pass.
If the M5S leaves the chamber, then the PDL should also leave the chamber. Their absent senators sum to 171, which means that the number of senators present cannot be more than 144, which means that the motion fails to reach quorum (158).
This is true whatever the Montiani or any senatori a vita do.
The only exceptions to this are cases where the quorum is lowered due to some Senators being on a leave of absence, or absent for reasons of tasks entrusted to them by the Senate (foreign delegations, for example). Any recourse to these, of course, would be very much una politica politicante.