You’re right that there are different styles, and the dynamics depend on the context. As Carl says, if you listen to the recordings, this will help you decide how loud or soft to play the “piano” sections. If there is thick orchestration, you must play the “p” more loudly, but if it is a solo, then you can show what a beautiful, focused piano dynamic can sound like.
(2) Harmonics in a fast and “ppp” passage.
It depends on the piece. For example, in Respighi’s “The Pines of Rome”, there is a section where the harmonics can be played as normal notes up the octave. This piece is not usually required in auditions. But in Rimsky-Korsakov’s “Russian Easter Overture”, you have to play the harmonics. (In this piece, I alternate left and right hand. I play the left hand harmonics with the same technique as the right hand when the harmonics get too high on the harp.)
(3) Rest in between measures
What is the maximum measures that you would count?
As you guessed, it depends on the speed. Don’t make the committee wait too long. Just make sure that you count the bar or bars in the same tempo as the rest of the passage.
Any good blog for some tip?
Google “auditions” and you will find many articles and tips on the Internet. For example, I have one on preparing for auditions. http://www.elizabethvolpebligh.com/articles.shtml#audition