Everything would be covered. You need to add seat reservations on French & Italian fast and high-speed trains (extra costs). Have a look at these 2 helpful links first :

https://www.seat61.com/how-to-use-a-eurail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

(Somewhat slow) wifi is available on high-speed trains, rarely elsewhere. Mobile coverage vastly depends on the area. Trains are running hourly on most lines, sometimes a bit less frequent sometimes more. For planning use the DB Navigator app (tool of the German railways connected to the European database). Rail Planner on the other hand is quite unreliable so don’t plan with it.

Now about your itinerary in particular :

Take the slow but stunning route between Zurich and Milan, namely the Bernina Railway. More details here : https://www.seat61.com/trains-and-routes/bernina-express.htm → either pay 36 CHF for the panorama carriages or simply board the regular unit (reservation-free) at the front.

A day trip to (the base of) Mont-Blanc isn’t really doable. The most famous part is on the French side, accessible via Switzerland and far away from Milan. I’d rather spend a day or two extra in the Swiss Alps!

Cinque Terre is perfectly on the Florence - Nice route so I’d stop there for a night or two along the way.

Lake Garda is a 50 min journey by high-speed train (13€ extra) or 1h25 by regional train services. This day trip is certainly doable.

Questions very welcome.