Quit chasing deficiencies, since you dont know what exactly is wrong you will only cause more problems chasing them.
Yep, the plants are entering senescence (plant death) and will continue to fade.
Exactly, they are pulling unused mobile nutrients now. That’s why they fade.
Again you should use the “in my experience” here, because it is a real thing. I’m just going to go with the old standby, “its cultivar specific”. Running an 8-week cultivar 11 weeks will not increase yield. They will continue to put on density up until 8-weeks though. Once fade begins, it is all ripening IME.