Mythos burr carriers do get jammed quite easily, even without being in a shed for that long.
Things to try:
I would probably use use some wd40 as well, because it will get cleaned off anyway
You can take the clump crusher out and you can push a flat screwdriver between the carrier and the backplate and try and release it (same with the burr and the carrier)
You could also remove the screws from the burr and put in some longer screws to try and push the carrier out from behind. Just keep tightening the screws slowly alternating between the 3.
Failing that a bearing puller is probably the best tool for the job, if you have access to one it can usually remove the most stubborn of carriers
