Arles, France -Historic center.

My partner and I stayed in a ETB recommended hotel in the historic center. Parking is possible but difficult. Also access is limited by bollards that require a code to enter some streets. Hotel recommended, wisely, to leave car near the river and abandon it while visiting Arles itself. The walk from car to hotel was under 5 minutes and worth not hassling with the small, twisty streets. The historic center is compact and is easily visited on foot.

Just checked out getting from Florence to Marseille. Consider taking the train to Milan and catching a direct flight to Marseille. Check out the route here. Seems like a lot of convoluted routes involving numerous changes and using buses and trains when leaving Florence . Some of the scheduled times are very inconvenient as well. I don't think the scenery is worth the aggravation.

I can't answer all your questions but a few years ago we rented an apartment in Arles for a week too. It was this one in case you have the same one, I can share some tips: https://www.vrbo.com/128269
The apartment didn't have parking but we found street parking nearby without many issues, then it was market day so we had to find an alternate plan. There's a parking deck next to the post office right outside the walls. It was super easy. I wish we knew about it from the beginning. It was closed on Sundays so keep that in mind if you are daytripping on a Sunday or just use the train. There's also parking near the train station but it was always full when we drove by. Grocery stores, bakeries, and of course the market day kept the kitchen stocked. Have a great time!