As far as I know there are no flights for which Interrail gives a reduction. However, you could use an overnight ferry, so you travel while sleeping. Grimaldi gives some reduction but there are more ferries from Sardinia.

A cheap plane ride to somewhere in mainland IT is most likely-if booked smartly (WIZZ or RYAnair), cheaper as a reduced ticket for the ferry. or-but that will cost a little more-fly direct to other country.

You are permitted 1 free trip OUT and IN on a pass in your own country-I have no idea if that would include say home-Aeropuerto/porto and then again on mainland to some borderpoint-IF yes, it must be done in 1 day, clearly impossible if you use ferry.

There used to be a ferry by then FS=the strike/state railway of Italy, to/fro Civitavechchia, that would even also take some train-goods-wagons, but I think this is history. On those boats you could use InterRail.

Nonethelss buon viaggio!

Yes, you can use Interrail on one day for the outbound/inbound journey and that can also be from home to an airport and then further to the border.

With a ferry, it depends. Since you don't need a travel day for the ferry, you can use that on the Italian mainland (but then you'd still need to pay any trip on Sardinia to the ferry port yourself). In addition, there are also ferries from Sardinia to France (Marseille, Toulon) and Spain (Barcelona), so you don't need to travel on the Italian mainland.

Ask directly the DiscoverEU team, if you can get the plane payed. I know, that people like from Malta can get by plane to mainland Europe and then go throw Europe with the interrail pass that they won with the train.