i would look at an X on the back going from side to side
lightly pin a bit off say 4x1" to the highest two points then another bit at 90% clamped or screwed to force the door over 50% the twist the other way
On a M&T door, if you put your foot against the bottom corner on the Lock side, with the bent top bending away from you, grab hold of the top corner and pull it towards you. There may be a crack, and the joints will move enough for the door to reshape, I have successfully done this on numerous old M&T panelled doors. The crack is the old paint in the joints. You have to pull it slightly beyond the vertical because it will spring back a bit.
p.s.
as yours is a stable door I suppose you will have to brace it against your hip or something rather than your foot.
yes so when you relax it it spring back flat hopfuly
its trial and error really
along with john d method
then you look at wetting adapting joints and adding a perminent timber with a counteracting spring to neutrilize it
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