Can the arm spin on the axle? I find it far easier to remove cottered arms from axles if there's rotational movement between them. I also will use a large "Crescent" wrench on the removed arm end of axle to hold it still. Have you applied a drop or two of penetrating oil?

Failing with this a next step is to remove the LH/adjustable cup and balls. Place a strip of leather along the chainstay where the now freely flopping about RH crankarm will hit it. Somehow support the frame while hitting the RH end of the axle with a punch and hammer and drive the axle out of the arm.

Failing that get your cut off disk grinder handy... Andy.