So last year me (been to Cannes several times) and 2 people who've never been to Europe are planning on staying in Cannes July 4-11. Were all vaccinated and it seems as if we will be able to enter by then.
The Airbnb is in Cannes. I found out the Film Festival will be happening at the same time due to the events of the past 1 1/2 years now. Having no interest in films, would the extra traffic and everything going on with the festival negatively impact our trip? Should we rebook an apartment in Nice? Or will it be barely noticable?

Thanks for your time and reading this!
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I don't know how it will be this year, but in other years it is definitely noticable! The whole of the Croisette is covered in boardings from film companies, most beaches have event tents set up for parties, restaurants will be full. In a 'normal' festival year I would certainly stay elsewhere during that period.
This year it will have to be low-key; even if you can travel to Cannes, there will be no large parties and gatherings.
There's a pretty strict lockdown in France now, I don't know if July will be OK for travel.

If you have no desire to walk on the Croisette or see the seafront and always stay north of rue d'Antibes, you should be okay. But most people visiting Cannes want to do the exact opposite.

Ughh the place we booked is 2 blocks from the Croisette. We have a couple trips planned for outside the city, Ventimiglia, Monaco, Maybe a day at Gorge Du Verdun. But my friends have never before seen France and it would be a shame if we cannot enjoy the Croisette while we're there. I just don't know if it's worth switching the Airbnb to a location in Nice, since I know we won't be restricted from going anywhere in Nice.

Frankly the spectacle is often fascinatng. I have flown down from Paris just to spend the day there in the past during the festival.

I just wanna know if we'll still be able to go swimming at the beach while the festival is happening. I know they will be cordoned off at night

If you pay for the private beaches, you can go.

I would imagine if you continue towards LaBocca, the free beaches will open again at some point, seeing as it becomes more suburban as you go.

But then you are not really in Cannes anymore....

As others have stated, the town is overrun during the festival, and even before and until just afterwards there are fences, walls, and temporary installations blocking La Croisette. Some fortunate few I know have scored a ticket, but those are rare. (I have a place in Nice and have been to Cannes at Festival time). Restaurants can get overrun and I can imagine that having this in the summer will make the famous beach clubs packed (and difficult to score a reservation). It is also expensive. I'd avoid that location unless you really want to be there for the Festival.

Even the restaurants on the other side of Rue d'antibes and the train tracks? Nobody in my party drinks or goes to clubs so we aren't worried about that. The majority of our trip we will be leaving the city to visit other parts of the riviera, but I booked Cannes as the base camp mainly because my grandma is buried at the cemetery there and I wanted to be close, and I don't know Nice. I know Cannes very well. I have simply never been there during the festival. I don't want it to turn into a miserable time for the two I'm bringing with me who have never been to Europe before, and I just want a way to get to the beach.

It will be different this year, I'm sure not everything will be back to normal. You can still go to the beach. In past years we had planned for a lunch on one of the beaches, but lots of places had special events going on instead. You don't really notice the festival north of Rue d'Antibes, as Kerouac says.
You know Cannes well, so you know how traffic to/from the motorway gets when it is busy. Since they made the Boulevard Carnot narrower some years ago it has not improved.

We stayed in Antibes during the 2016 festival and actually went into Cannes the evening of the inaugural. Later that evening, we walked along the Croisette and up to the festival center, very few crowds at that time, but it gave us a thrill nonetheless.

We visited Cannes one other morning, got there early and walked around for a few hours, then left around noon, when quite a few people were up and about. I can imagine it’ll get pretty busy though, especially restaurants, etc. We weren’t interested in the beach, so I can’t tell you anything about that.

That sounds like it will work then. We are right below Rue d'Antibes so if everything is still relatively stable/typical July above then it is no problem. I understand that it gets busy in the summer regardless. I was there every year between June-July every year for 10 years and it got very busy with the typical tourists. It was manageable. I have never been there during the festival. Sounds like the worst possible roadblock will be beach access, and there is always the option to taking the train 5 minutes to LaBocca. Also the boat to St Marguerite, I would imagine, is running.
It's been 15 years since I've been there and Boulevard Carnot was always packed. Why on earth would they make it narrower?

Probably because most of the cities of France are trying to make their streets less friendly to motor traffic.

Oh wow. That's difficult for me to understand. Here in Philadelphia we have public transit, but if you don't have a car you will still be limited and have a hard time. I know parking is difficult in Cannes but we haven't yet decided on renting a car for a couple of days to make day trips to certain places, like Italy and maybe Gorge du Verdun, easier. I don't even know if we'll have time to do the Gorge.

Parking is easy in Cannes, there are plenty of car parks. The largest is under the Palais des Festival, and maybe not accessible during the festival. Not much free parking, but I don't think you can expect free parking in a city. The train is easy for day trips. If you want to go to Monaco take the train. You can also hop on a train and go to the beach in Juan les Pins.

I was just going to say, just take the train to Juan les Pins (or La Bocca) to go the beach (I'm not a fan of La Croisette, anyways). It is a new timing, COVID and a reopening at the same time. No one truly knows what is going to happen, but if you are out of the primary areas it sounds like a good bet you will be OK.