How about giving users the power to control the stemming process? One would be to respect quotes, that for any word enclosed in quotes, stemming will not be applied. That could resolve @BmB23’s situation.
Another would be for search operators to override stemming, like the aforementioned example of “search engine -engineer”. That could help mitigate the problems of stemming through elimination.
Even more powerful if Mojeek could implement a checklist for words it guesses through stemming. Either a horizontal checklist above the search results, or a vertical checklist utilizing the free space on the left, like for example:
engine
engines
engineer
Then the users can manually uncheck those that are wrong or are not applicable to their search. This would also be a huge boon to feedback, where users can easily report back all wrong predictions.