If unseen swelling is a problem, couldn't you fix it with a simple sensor, like a fragile paper tape conductor that goes around the battery?
It swells, the circuit goes dead, and that's the signal for the charger to stop charging.
It swells, the circuit goes dead, and that's the signal for the charger to stop charging.
Once the layers of the cell make contact it shorts the entire battery and causes the battery to discharge quickly, releasing the energy in the form of heat.
Just because you've removed the power source from the battery doesn't mean the battery won't stop discharging.
In my experience Apple is not consistently good about dealing with this. In every case, I've ultimately been able to get the battery replaced at no cost to me, but with an iPhone 6 that started swelling about a year ago, pushing the screen out, phone support told me flatly that this wasn't something that could possibly happen with such a new device, and suggested that I try to make a Genius Bar appointment on my own. When I brought the phone into the store later that day, they replaced it on the spot, with no appointment. (I wasn't going to accept carrying around a time bomb for another week or more.)