To your first point, I doubt they will group parties in the middle of the day when it is hot. This thing is going to have massive capacity, i.e. a full PLANE load of people every 2 minutes, each way to each park.
As far as getting stuck, I think there will probably be emergency windows or door release so you could get some air and not bake. I tried to find a some pictures, but haven't been able to.
I find it doubtful that the gondolas can load a plane full of people such that there will be little to no wait. The London gondola, which is supposed to be a similar model as the Disney one (2500 capacity per hour each direction), is described by some online reviewers as follows:
"We visited this on bank holiday Sunday and wished we hadn't bothered - we were told by one of the staff that the queue for the tickets was about 5-10mins - that was a lie it was nearer 25mins and then you had to join another queue to go up to the cable car itself - that was another 10-15mins wait."
"We had to wait in an enormous queue to get to the cable car. It was probably around 45 minutes wait."
"Bad Customer Service. Huge Queuing Times. rushed approach to load you into the cable car."
"Waited 15 minutes to ride. Ten minutes of which was because the attendant was chatting to his mates while several half empty gondolas went by."
"Bought Viator tickets for river cruise and cable car. Excitedly followed instructions to ticket kiosk. Queued for very long time. Presented voucher and told we could not go as cable car closed for 'planned maintenance'"
"The worst complaint is that you cannot simply stay in a cable car for a full round-trip ride (not explained before boarding). You HAVE to get off at the end and re-join the long queues even if you want to come straight back. Even on the windy/drizzly afternoon yesterday, the queues weren't short. I can't imagine how long they get when the weather is fine and more people deciding to try out the cable cars."
"Oh, by the way, if you get stuck up there you are being evacuated onto a boat, straight down. Just so you know."
"You're more likely to queue up for longer than it takes to cross the Thames in the cable car which is approximately 20 minutes."
"The return trip later that day involved a 20 minute queue to get on though."
"there didnt seem to be a quick and efficient way of loading people into the cars when it was really busy (they need some tips from disney!)"
"There was heavy wind on the day so the cable car was closed we waited for an hour then decided to give it a miss."
"The queue was very long as expected,but it does move fairly quickly so you're not having to wait an excessive amount of time."
"Show up early in the day. The cable cars are slowed down below what you'd normally expect from them to make the trip longer - fair enough for having more time for the views, but it leads to ridiculous queues."
"The queues at the weekend can get very long as they are slow moving."
"The cars gold up to 10 people and you are expected to share with strangers to fill the car to capacity. Of course we ended up with the guy who thought it was funny to try and rock our car."
"We queued up for about an hour got on the cable cart and it was over within about 10 minutes and when we got off at the other side there wasn't much to do at all so i was disappointed with that."
"Then a little wait to actually get on the carts too."
"Each car can hold up to 10 people, so it's quite likely that you will be sitting with strangers, even if you are part of a group. I found that the staff generally did not fill the cars up, but sometimes one or two riders would push forward and jump in a car that had only 5 or 6 people."