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The leather washer has a hole in the centre (like any good washer), put a small "L" bend (slightly 'over' an L) in the end of a wire coat hanger and you can hook the washer out as 'easy as you like'.

Or, a wire coathanger with a small hook bent into the end.

Well done - it just needed the right tool for the job.

The 'head' sits on a 'ledge', below which should be a metal washer.
The shank is a 'push fit' thru the butt out to the butt socket. (so yes - the shank is also a 'tightish' fit)

Just a thought -...

5mm Head depth

I would try to avoid the heat and go for the 'socket' first.

If the forend is removed you could heat the end of the butt-bolt where it comes thru the butt-socket, but personally I'd be wary of...

The actual butt-bolt is very roughly (I cannot get to my Calipers at the moment)

Shank 12mm diameter,
Head 18mm diameter (so up to an 18mm wide screwdriver will fit)
Slot width 3mm
Slot depth...

Penetrating fluid will only work on a metal bolt seized in a metal 'hole'.
What you have probably done now is actually made the problem worse by giving the wood some more 'liquid; to absorb and...