I don't know who said to cut it because it would spread... WHAT would spread? Don't worry about the leaf or leaves you've cut, I don't think you ever had anything that "could spread".... What I see is a plant that needs some water... and the spots on the leaves you have pointed out here are the result of some water droplets getting on the leaf and burning the leaf from the lights...
The problems you're having with this grow come from feeding this lady much too much and much too early. Autoflowers don't like a lot of nutes and certainly don't like nutes before they're at minimum 2 weeks old.... You need to give her only plain pH'd water for at least a week now and then reintroduce the nutes but at 1/2 strength... And don't go by the feed charts of the nute company - those are written for people growing photoperiods, not autoflowers... I've never seen an autoflower that can tolerate full strength nutes from any company! With autoflowers, less is more....
Good luck!