Admob fill rate and Mediation networks

I am using new admob interstitials…


i have 2 questions question is whether a request is counted when we request for an add or when we try to display an ad.
I mean,in my app i pre-load interstitial ads,and display them when user does a particular action.So if the user do not do that action,the loaded ad is not displayed.
so in this situation admob counts a request and shows no impressions,right?

2.thinking of using mediation networks for admob interstitials.which one is best.

Thanks a lot