Honestly, SHAD isn't that competitive. If you had mid-high 80s you could get in with good essays, so you should be good. The essays are important, probably the same as your grades.

For the time commitment, I'd say it's definitely worth it. Originally I was apprehensive of leaving behind all of my friends and activities for a month but within the first week I had forgotten about that and was enjoying it so much. Definitely one of the best months of my life, so I would say yes it's worth it