1)depend how you want to setup the monthly plan. You can read the ARBUpdateSubscription in the documentation, but there isn't a easy way to do it in the middle of a schedule interval.
If you going to setup a new plan starting on the 15th? do a AIM credit or $10.
If you going to change the plan to $50/month but keep the same date as 1st. do a AIM charge for $15.
2)As long as it for the same company, you can use it for mulitple sites.
3)Yes, while ARB run on your backend server, the website will need SSL for customer to enter their credit card info.