Barcelona to Rome over 14 days in July

A one-way car rental from Spain to Italy would be very expensive with the extra drop fee. You could always pick up a car just over the border in France and the drop it off in Nice or eastern-southern France and take public transit (train, bus) to just into Italy and pick up a second car rental there. We did one-way car rentals in France and Italy with no drop fees. We also trained from Nice to Genoa, but had to change trains at the border due to different gaged trains.

A quick look at car rentals shows a one way drop charge ( on top of the regular rental rate) of ~$1000.00

With 8 people and luggage you likely will need two cars. I did find a 9 passenger van with a drop fee of $1800.00, but it shows luggage capacity of 3 bags

You may want to re-think the one way car rental.

2) Is it safe to assume most or all AIRBNB's would have air conditioning units?

I would not 'assume' that to be the case. I would research that point carefully

Thank you both for the responses. I knew there might be a drop fee but had no idea it might be so costly. I think the idea of separate car rentals in separate countries is a winner. Now I need to look into drop fees within the same country but in separate towns. Perhaps we do public transportation to each "home base", rent a car in that city and return at same agency to avoid drop fees altogether.

Drop fees within France or within Italy are practically nonexistent with major rental agencies like EuropeCar. I usually start my search at www.autoeurope.com, a US consolidator that represents several major agencies (but not a comparison of all possibilities that exist). Trains run all along this coastline, so you can easily connect train and car pretty much anywhere. When choosing car rental locations, "train station" is a common descriptor/option.

The fastest international trains departing from Barcelona can take you directly to Perpignan, Narbonne, Beziers, Montpellier, Nîmes, Avignon, Arles, Aix-en-Provence, Marseille, or a couple of other stops.

I would not assume that all accommodations have AC because that's setting yourself up for potential disappointment. AC units that hang on an interior wall have become common, but confirm as you book. Some owners might assume that you can just open the window to let in a coastal breeze at night (plus possible noise and bugs). During the day, leave shutters closed to block the heat.

There have been reports that Barcelona wants to do something about the conversion of so much of its housing stock into tourist apartments. Check the status of any possible,pending local legislation carefully. With 8 people to house you do not want to have a rental cancelled at the last minute for legal reasons.

European beaches tend to be much, much more crowded than US beaches due to a combination of greater population density and the tendency of sun-starved northern Europeans to head south in July and August. Do some research about the specific beaches where you plan to spend your time to avoid disappointment, especially focusing on whether you will be lounging on sand or on rocks.

I'm not clear on whether this is your first trip to Europe. If it is, and you see this as primarily a beach vacation, I would gently suggest that you might do better in a different part of the world.

....Drop fees within France or within Italy are practically nonexistent with major rental agencies like EuropeCar.....

They may be practically nonexistent but they do exist so don't assume anything. We were tagged 50 euro from dropping a Hertz in France last year. It wasn't a big enough fee to be a deal breaker.