OP - you are making this more complicated than necessary.

First of all, as Italian citizens you have a right to be in the EU for as long as you like.

Second, when you are at the airport, you will have a passport check when you leave the Schengen area. If you show only your Brazilian passport the officer will probably ask you why there is no entry stamp. You can reply that you entered with your Italian passport. You will then show this document (or you could just show that to the officer in the first place) and you'll be on your way. You may or may not get a remark that the passport is expiring soon. You can of course renew it at an Italian consulate in Brazil. That will be the end of it.

When you are at the gate getting on the aircraft, you will show your Brazilian passport.