Hello matt,
Let me answer your questions one-by-one:
Q1. Can we admins assign users to a paid plan, without payment? (for the user that have already paid on the old system)
Answer: Yes, you can manually assign the specific membership to a user.
Q2.1 - From what I read - it seems that when the time period is up (say 3 months) The user stays on the same role and the admins have to change the role manually?
Answer: This is how the core User Roles system works. This system has not any period. But if we take up the Paid Membership system, so then, you can set the specific period for a membership and after that period the membership will be taken away.
Q2.2 - When using with the paypal plugin - the user is signing up for a recurring payment, not a one off? Or can the admin choose? So for the user to quit the site, they must cancel the recurring payment, and the admin must down grade them manually?
Answer: This functionality has nothing to do with the payment processor you are using. All depends on the membership plans settings. You can create a single plan (the membership will be taken off after the chosen period expires). Or you can create a recurring plan (check the 'recurring' checkbox) - in this case a user will have to manually cancel the subscription on their PayPal account in case they decide to leave the site.
Q2.3 - If using a non-recuring payment (ie. One off) does the user role change back to default at the end of the payment period?
Answer: Yes, the role will be taken off as well as the membership.
Q.3.1 - Does the user or the admin get a PM or Email indicating activity? Eg. When a membership period will expire (say 5 days before?)
Answer: Admin can configure the reminder about the membership expiration in the plugin settings. For example, admin can set 3 value (days) for the 'Remind users by email that membership expires in'. Then a user will receive a notification about that their membership expires in 3 days.
Q.4.1 - Can we use more than one payment provider simultaneously? Eg Paypal And CCBILL?
Answer: Yes, you can use several payment providers. All of them work independently and should not conflict with each other.