If you're dead set on trying to repair that, about the only thing I could recommend is to cut the back 5" off the whole back edge of the top (using a thin kerf blade). Remove the veneer from he cut piece, replace the partical board (with a solid board or plywood?), then reapply the veneer to the new base board.
Remove the back panel, replace any partical board supporting that back panel, similarly as above, if possible, then either reattach the panel or replace it with a new panel (1/4" or 3/8" luan?).
Trying to clean up that defected partical board is an absolute waste of time, in my opinion.