"I presume this is a non-flexible porcelain coating vocalizing its displeasure with flexing."
Metal, esp. coated metal, is not like applying pressure to say, wet wood to then get a finished curve for furniture or something.....
Bowing a lid back into round should take maybe 3 minutes, tops....most of which is figuring out which direction to bend......once bowed, it stays.......
I did the 1961 Fleetwood 26" in a minute or two & it shuts down perfectly now.....
Put one side against your belly, hold the opposite side with two hands at 11 o'clock & 1 o'clock & give little short pulls / squeezes, gently......should then not spring back........the clamps are merely holding the spring in place until they let go & then spring-loaded metal does what spring-loaded metal does......