The shelf should be mounted high enough to clear the faucet operation, and then the mirror high enough above that to not impede anything you would normally place on the shelf, but low enough to be of service to whatever height people who will use the bathroom. As you mentioned they will all be installed above the other, then the 3 bulb wall light would need to clear the top of the mirror, and not hang so low as to (once again) impede use and opening/closing of the mirror. If this is not a powder room, you may wish lights on the side of the mirror, as this setup usually produces better lighting. Backsplash tile height/placement is harder to assess without a visual. Suggested to mock it up on the wall, and if it looks off, to re-assess.

Thank you - do you think the shelf should be as wide as the vanity counter 24" or 14" wide under neath the 20" wide medicine cabinet?

I'm sorry -- no idea on appropriate width. The previous suggestions were based entirely on practicality -- have to be able to use the faucet, open the medicine cabinet without knocking shelf items off the shelf, and without hitting the lights on the fixture. If the placement height of the shelf was determined, you could purchase/hang/mockup the mirror, and then decide what width of shelf would look best. Good luck!