I actually never knew about undergraduate journals, thank you for pointing these out to me. Do you have any publications in Physical Review A or Physical Review Articles via the work you did with your PIs?
I do not think your GPA is an impediment at all. Given that it is at an Ivy, I think anything over 3.5 is very impressive. This is just my opinion though, I'm not sure. I just think, your 3.7 GPA at an Ivy will look much better than my 3.9 from a small high admission rate liberal arts college.
Yes absolutely, your summer PI will make a great letter. My adviser over my junior year REU was pivotal in my success. This will be especially true if your adviser is well-known in a field you intend to pursue in grad school.
No question, you should retake it. Take your time over the summer to expand your comfort level with a variety of topics. What worked the best for me was, after studying all summer following the study plan I link below, was learning the 2008 and 2017 back exams to a tee, i.e. memorizing every aspect of the problems.
Study plan I will be following (includes old pGRE tests):https://sites.google.com/a/uw.edu/physi ... est-gr8677
The "essential text" although I hate it:https://www.amazon.com/Conquering-Physi ... TS3MQ8H7WW
Go-to intro text for literally everything (free online pdf):
google "halliday and resnick fundamentals of physics pdf"A supplementary text I love:https://www.amazon.com/Physics-Student- ... wry+kirkby
Article/website with good info:http://www.alexhunterlang.com/physics-gre
After you have gotten through all the practice tests, use this to study specific areas (I say after because these problems are from old tests):http://www.physics.ohio-state.edu/undergrad/ugs_gre.php
Email this person(s) at Case Western Reserve for free online flashcards:http://physics.case.edu/flashcards/