And is it ok to comment on students proposals in a shared google doc that only student and mentor have access to before the students submit their final proposal via the https://summerofcode.withgoogle.com/ site?
The 10 questions should be clearly answered and numbered, but otherwise the format is up to the student and mentor to ensure it clearly describes what the student intends to do. See also the other guidance also in the “Application Requirements & Questions” section you linked to.
The Google submission site works on Google Docs, so yes, it should be possible for a student to pre-submit a Google Doc there for review & comment. (I haven’t got around to testing/verifying this yet.) But at the end the student will need to convert the doc into a PDF for submission.