For #5, I broke the last sentence down by identifying "can't" as a Group 4 negating necessary implicator. Was that wrong? Technically, the Group 4 indicators are "none", "both", "not both", and cannot", so I wonder if my mistake was assuming "can't" and "cannot" were the same.

The video says there is an implicit "all" before spellcasters, but that is not something I thought about when working this question out.