Hi Gretchen,
Here are my thoughts on your questions
So, regarding the tooth – from the sample nurse builders questions yes rinsing in tap water is ok
Impetigo- I don't think they will get to specific on cause just signs and symptoms, time contagious and treatment
Test center- I just scheduled my test for August 5th (gulp!) Test centers usually have a check in process for people taking exams. Usually they check your ID, assign you a storage space (locker with key) check your pockets and then take you to your assigned computer. It's like taking the NCLEX.
As for percentage of practice questions I think Janice Selekman answered that with there really is no data to answer the question. Mostly because everyone uses different study resources.
I'm in Connecticut where are you? I am using the online study group, the nurse builders course, the review book (currently 2020 edition) and the Comprehensive textbook. I plan to purchase the NBCSN practice exams and may get the updated review book.