I would try with hair dryer, on a wooden workbench
In moving hair dryer a lot, to avoid plastic becomes too hot at the same place.
With hair dryer, it is not too hot ( like could be heat gun )
It depends also on the distance between the hair dryer and the base plate.
Then adapt temperature of hair dryer ( don't use maximal temperature ) and distance
You need to stop frequently , to check how plastic reacts.
Heat and stop, touch plastic, heat and stop, touch plastic ( it can be hot )...
You have to be patient, patient and not try to heat up quickly.
I straightened out this way some Playmobil, for instance wagon roofs that were warping.
Go slowly and stop if you don't feel at easy.
Sometimes this deformation comes from to stay under sun during summer in a room like a green house
It is to say that hair dryer does not need to be very hot....
Do not try with lighter.
When I was younger
, I was in too much of a hurry, with heat gun,
I twisted this door.
But it was my first try, and it it was very bad plastic.
Normally Playmobil or LGB plastic is stronger...
Hot water is really good idea
but , in my case, it is easier for me to stop hair dryer than to remove roof from hot water...
