Honesty should be assumed in the warranty claim, if you’re sure it was hot then simply state it.
If against our advice you feel you need to prove it again, do it outdoors in a ventilated area.
The voltages are low so not particularly chance of harm from electricity but more from chemical vapours or fire
Heat implies there’s a cable short in the end of the cable being plugged in, or in the port in the headphones. You’d have to have a higher current charger of a lower quality continue to drive maximum current through a cable with the end of the cable in the headphones shorting, or the port has a short, for it to get hot. That heat is near to a Lithium battery which will bulge and eventually set of fire.
Probably bad cable, or debris in port, or rarer is charger issue not regulated, one of those is Q30 fault. As you stated you tried different charger including 5V 1A it infers more probably the Q35 has an issue in the port.
Let us know if warranty claim are not accepting your assertion of heat.
It’s possible you did damage using a very bad charger which didn’t regulate, it melted metal in the Q35 port causing short so then a different charger still drove 1A 5V 5W through the short. A photo should help eliminate prior bad charger fault.