Can you fashion a hook to insert into the curved opening in the macerator housing... and then pull there?
Or... can you access the impeller set screw? Maybe removing the macerator housing and impeller all at the same time is possible?
Or... perhaps some kind of spray lubrication -- where macerator housing meets the impeller sides? -- coupled with some "gentle" tapping around all the sides, for a while, might help eventually?
Or... a pliers-like tool with really big jaws... or maybe even a household lid-opener (for lids on glass jars)... might let you get enough purchase
on the housing so as to be able to twist/rotate it a bit... which might loosen whatever is holding it?
When I rebuilt ours, it came apart as the instructions describe...