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.

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

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.