Facebook Ads: Preload an ads in scene and show it in another scene

Hello,

Question 0:
I use FacebookAds and I want to know if I can (it is allowed ?) preload multiple ads using class with static variables and fnc.
It's for load some ads in scene and show them in another scene.
What I want:
I lanch my game and arrive in the menu scene, I initialize my static class, and then I can load somes different ads in static variables, keep them and show them in "EndGame" scene after loose.
How much time loaded ads stay alive ?

Question 1:
It is recommended to use multiple ads network ? FacebookAds have enough ads to show or I need to help it with another network like UnityAds when no more ads are available ? How much network is recommended ?

Question 2:
It is recommended to reward people only when they click on the ads "Install button" or other ? I think it give more money but I don't know if it's allowed... And I think I have to explain them to click on the button to get reward I don't know if it's good to do that...

Have a nice day