Reservations fees on the "fast" trains are usually compulsory. These fees are not covered by the Eurail Pass, so you'll need to pay separately. I'd suggest finding the nearest SCNF Boutique or large rail station and arranging the reservations as soon as you arrive in Paris. That way the details will be out of the way and you can enjoy your sightseeing.
Regarding some of your rail journeys, I checked briefly using an arbitrary date, and these are some trains you might consider:
> Paris to GImmelwald - there's a train departing Paris Est at 08:24, arriving Interlaken Ost at 13:57 (Time 5H:33M, one change in Basel, reservations compulsory). That's a really good travel time, considering the distance travelled! Although I only checked the route to Interlaken, the travel time from there to Lauterbrunnen and area is not significant. You can buy a ticket at Interlaken that will cover all modes of transport to Gimmelwald (train, Post Bus and Cable Car from Stechelberg).
> Gimmelwald to Cinque Terre - this trip is a bit more complicated, but still reasonable by train. The trip I'd probably choose departs Interlaken Ost at 08:01, arriving at La Spezia Centrale at 15:18 (Time 7H:17M, 3 changes in Spiez, Brig and Milano Centrale, reservations compulsory). Each of the changes is less than half an hour, so not too bad. I searched using La Spezia Centrale as the destination, as you didn't specify which of the five villages you were staying in? The local train runs to the villages frequently so that shouldn't be a problem.
You indicated that you'll be travelling from Rome to Paris towards the end of your trip. That trip would probably be better via budget airline, as the rail trip will be about 16 hours, with perhaps one change in Milano. You might check flights with Vueling from Roma FCO to Paris CDG.