Thank you for your interest.
1. Yes you need to have a ChargeBee account. A major advantage of using ChargeBee is that since we are an online hosted solution, so you don't need to worry about any of the security aspects when collecting sensitive information from your customers.
You can integrate your existing Braintree payment gateway account with ChargeBee. You can also migrate over your existing subscribers into ChargeBee and we can manage them. Braintree will remain the payment processor but ChargeBee will provide the additional sophistication to seamlessly take care of your subscriptions, invoicing and billing needs. Here's how.
2. Yes ChargeBee supports one time charges. The ChargeBee plugin for WordPress was released last week and we are working on adding a lot more options to it. If you could let us know your requirements and possibly a mockup of how your one time charges would be, we can work together and improve the plugin to include it.
For questions email us at firstname.lastname@example.org.
We also have a customer portal feature for your customers to manage their subscriptions themselves.