Peter,
your trouble un-installing Formulas 3.0 (it would have been the same for any old question type made for Moodle 1.9 or 2.0) have been properly documented by somebody 
in the tracker, this is MDL-31099 Better management of errors when an old (incompatible) question type is installed, but as it has only 2 votes, I guess it's not a priority.