DS did this last year at nearly the same date. +1 on school guidance counselor sending PDFs and/or detailed explanation of documentation via email to admissions counselor-do it this way even if the updates are not school-related. If counselor can include a brief description of significance of new award/leadership position etc., that could be helpful.
Yes. Any additions must be "endorsed" by your school official. I suppose items outside the school where there is "objective evidence" (e.g., certificate/award from the organization) could be send without an endorsement, but I'd still take that to the school official and get the submission endorsed.
At this point in the year, the update should be "significant" -- such as receiving an award, being elected/selected as varsity team captain, making all-district/all-state in athletics, etc. Doing more community service, participating/joining another activity, etc. is unlikely to make any difference. However, if you feel it's important, send it in.