1. It's 'can cause' since it's still suggesting a possibility, and as personal preference, you can add "or not" after "whether." Or not. HAHA
2. 'ever' is fine, while "can't and couldn't" depends on the time when the action was done. Can't is present, while couldn't is past.This is usually determined by context, so the choice is dependent on previous sentences, like if the son was retelling the story then it would be couldn't but if it is while mother and son is talking, then it is can't. Again, you can use "or not," or not. Hihi.