We bought an air dream for our pull out sofa in the TT....guests have said it is very comfortable and you don't get that bar in the middle of your back like you do on those paper-thin mattresses they put in fold out sofas.
Not really possible to over-inflate the mattress....the pump is more like a fan than a compressor, it is designed to get air in there quickly, not pump it up to high pressure like a tire. The reason the pitch of the pump sound changes is because the mattress is full and the fan can't push anymore air in. Like putting a towel in front of a hairdryer....
You could leave the pump on the mattress for an hour and the mattress isn't going to get any "fuller", there's no way to over pressurize it with that pump. That's why they say to take it off when the pitch changes, not because you risk any damage, but because it's done moving air inside and leaving it on any longer isn't going to do anything.
Not sure if you have tried it yet, but the mattress is plenty firm with that pump, guests have raved about how comfortable it is. Easy to fold up in the morning too, just pop that big hatch and it deflates almost immediately.
We have never had any leakage or leak-down either....the mattress is just as firm in the morning as it was the night before. We too have used those cheap-o air mattresses back in our tenting days, and they leak down not entirely due to the overnight temp change but because of seepage at the valve.