That is no problem and you don't need a reservation on the train if you avoid the panoramic coaches on the scenic trains that requires reservations. 

The outbound/inbound rule gives you the possibility to travel in your country of residence on a maximum of two travel days. You can take as many trains as you like during the travel day.

This is simply the normal route that was used by all treni before the new base-tunnel opened-it still has hourly local/regional treni -and going to IT this is always the logical normal route.

Only those fancy panorama cars need to be paid for extra- Swiss are known to be frugal/frankli saving so they would leave that to the foreign tourists who know no better

Thanks both for your replies! So basically it means I may use my Global Pass as a day ticket for two days and travel around Switzerland as much as I like even without any intentions to leave the country? In that case I'd like to use it for the Glacier Express to Zermatt.

I wasn't aware that the Gotthard Panorama Express has "non-panoramic" coaches with seats that don't require reservation. Even for first class? Because if I look up the GPE on the SBB app (Flüelen to Bellinzona, departure 14:09), it says "Platzreservierung obligatorisch" and "Zuschlagspflichtig". 

You need to go with the regional trains that take the same route as the PE. It will take about 30 minutes longer and some more changes.

Glacier Express and Gotthard Panorama Express have no cars that can be used without reservations.

Bernina Express has at the front of the train a reservation free section. Bernina Express Bus Tirano-Lugano needs a reservation. Tirano would be in Italy, so it is also ok with Interrail rules.

For Glacier Express you can use it like AnnaB shows in the screen shot. 

For Gotthard Panorama Express you can use IR26 or IR46. For the ship I don’t know, some are part of Interrail. Here a map of all services included in Interrail in Switzerland: https://www.interrail.eu/content/dam/pdfs/SBB-Interrail-Eurail-S-20-WEB.pdf

@Angelo You get a 50% discount on the ship from Luzerne with Interrail. I asked at the ticket office of the ship company when I was in Switzerland two weeks ago.