I would strongly caution against getting a LOR from someone just off of their title alone. You want someone who can attest to your abilities ideally as a student, but could also be as an employee if you’ve been in the workforce for a while. Almost all schools say the same: TA’s, professors, direct supervisors are the best bet. Fancy titles that weren’t your direct boss usually have a preprinted LOR format that cant give much detail and give crappy LORs. Your summer school professor, or even better a professor who’s class you took multiple times, and did well in is a great bet! And keep in contact after because you never know when you’ll need to network. Any specific questions about your situation, most schools usually have q&a seminars.

I am writing a GPA addenda for my own situation, but from what I gathered after speaking to admissions officers is to bring as little negative attention to your application as possible. The point of the addenda is to get in and get out. I need to explain my situation because it would leave a huge gap in my application, however if your situation is not glaringly obvious I would not. I would address personal growth in essays and put a positive spin on it. If you have any doubts I would talk to the admissions advisors for the schools you're looking into. Best of luck, but you wont need it :)

I cant say for certain, but the Oct test scheduling opened at 9 AM PST or 12 PM ET so I would imagine that would also be the case for the November test. They sent an email explaining the times the week before so I would expect an email from LSAC probably on the 20th with details.

Don't cancel it. If anything it just shows that you were able to consistently score in the mid 160's.