I'm no one to answer any of your questions regarding your c41 processing.
I must congratulate you on the quality of your color.
But the spotting on the negatives I know is your next quality control project.
You know where the problem lies with the gritty residue.
Now your mentors will assist you there I'm sure.
I'd celebrate with a swig of wine... crack open the vintage wine when you have everything under complete control!