Hello 

1. No, system allows a user to pay again. I think the user should know what membership level he currently has. 

2. In case of purchasing a membership level UNA waits a paid period of time + some number of days 'Reserve (in days) to accept and process recurring payment notification' to accept notification from Payment Provider about the next payment. If the next payment wasn't accepted during this time the license (membership level) will be canceled.

2.5. When you are creating a new pricing option (in Studio -> Paid Levels -> Manage) for membership level you may use 0 in Period field to be able to sell a lifetime memberhip. 

3. Stripe, ChargeBee and PayPal take the payment automatically and then notify UNA about accepted payment. UNA just save the data and prolog the license (in Market) or membership level (in Paid Levels).