Traveling by Rail from Florence -Lucerne- Interlochen-Jungrau- Lausanne- Nice

First add the costs of point to point tickets and if it is less that of a rail pass then go with that.
Also and you may know already that even after you buy the rail pass you will have to pay reservation fees on certain trains:
http://www.eurail.com/en/plan-your-trip/guide-eurail-train-reservations/reservation-fees

French trains have capacity controls on reservations for rail pass holders. It's possible to choose a train with plenty of empty seats and not be able to buy a reservation on it because you're using a rail pass.

If this is your first trip, you might want to take a look at the many photos on the Seat61 website. Most of us feel that 1st class is totally unnecessary. Second-class cars on European trains are generally very comfortable.

I suspect that you might benefit by purchasing some sort of Swiss travel card and buying point-to-point tickets for the other legs of your trip. Prices can be quite reasonable if you buy non-refundable/non-changeable tickets soon after they go on sale for your travel dates. You must be sure of the date/time you want to travel, though.

The Eurail pass only gives you a 25% discount on the rails between Interlaken Ost and the Jungfraujoch. You would be better getting a Swiss 30-day Half Fare card which gives a 50% discount for that and every other train, lift, bus and boat in Switzerland.