1- Yes, you need a PRO account for the ads and you need to use AdMob.
2- Yes, you can. You will share the revenue with Google only.
3- You will need an AdMod account and you will need to assign Thunkable to manage it. Thunkable will take part of your ads earnings.
4- There are other ad providers but currently Thunkable has AdMob component only.
5- IDK
6- You are allowed to distribute android apps but not iOS apps.
Hope that helped.