I removed a very stuck shock mounting bolt on the swing arm of a Marin.
Like your's the alloy spacers either side of shock eye were corroded to feck and stuck.
Liberal spraying with WD40 plus leaving to let it soak in overnight with more applied before I then tried to remove it.
I took off both nuts then put several washers onto end of mounting pin on one side leaving a nuts depth of threads.
Put the nut back on and started to tighten it to pull it one way, when it moved sprayed more WD40 so newly exposed part of pin was wet. Transferred the washers and nut to other side then kept repeating this until I could pull it all the way to one side and out. Took time and effort to get it to move more than a few mm at a time.
Just make sure you haven't mashed the end of the threads on one or both ends of pin because then you're never going to get it through the spacers or shock eye.
Extreme violence then required to remove it.
Yeah I got it moving but the extreme violence was too much for the old alloy and its now cracked!
You still need to get the mounting pin out to get the shock if you want it.
You could always use Mr Angrygrinder to cut away the swing arm, spacers and pin.
There's a certain satisfaction involved in saying "beat this ya bastard".
Not an option I'd like to use, too much chance of damaging the shock so I would persist with the nut and washers easing it side to side.