Renting a Car in France, Italy, Switzerland, Germany (drop off)

It is possible, but expensive. Drop-off charges can exceed $1,000. I suggest you have a look and get a quote on Autoeurope for example.

Possible, but as mentioned above, extremely expensive drop off fee. Also, depending on where you are in France, some rental agencies are not open on Sunday. Be sure to check which countries require you to have a vignette.

I've been able to pull this off a few times (2 or 3). It can take a lot of poking around with the various rental agencies. Generally, I'll rent a car in one city (Zurich) and plan a loop itinerary bringing the car to the pick up location. However, my most creative rental was Krakow to Dubrovnik. This rental required a pickup in Vienna (versus Poland). It was a 5 hour drive on the arrival day but it was really worth it to have the rental for three weeks and an easy drop at the Dubrovnik airport. The drop off fee (10 years ago) was approximately $200.
Good luck.

Note that to avoid large drop off fees, you need to return the car not to the same location where you rented it, but to a location in the same country.

Note also that international drop off fees can be very high. But also depending on the agency, the time of year, and the specific countries and rental locations between what you are traveling, drop off fees can be manageable. For instance, I've paid about $150 to drop a car in a different country. It's highly unlikely that you'll find reasonably low international drop off fees, but it's always worth taking a look, you never know.

Or maybe it's worth it to you to pay $600 (or whatever) to drop the car in a different country. Cheaper than flying a family or group probably, or maybe the itinerary you want is what you want and it's worth it.

Point is don't let the fact that expensive international drop-off fees exist categorically stop you from ever considering renting a car from one country to another.

Also note that if you rent in a large country You can plan a fairly convenient itinerary through a whole bunch of other countries and then drop off in a different part of the large country. For instance pick up a car just across the German border from Basel Switzerland, and then tour Switzerland France Luxembourg Belgium Netherlands before dropping the car in northwest Germany.

Or more directly to your itinerary, Nice, Northern Italy, Switzerland, Germany return in Alsace France.