I want to sell access to secure zones using webforms and will collect payment using a seamless payment gateway. What I'd like to know is whether the gateway needs to support recurring billing even if I've set the "Frequency of membership renewal" to "Once" for each secure zone. In other words, there won't be recurring payments collected.
No. If you just have a seamless gateway and on the form put a payment option and add a secure zone, you don't need a gateway which supports recurring payments. The customer will be subscribed to a chosen secure zone after successful payment. Make sure you select the actual secure zone and not "Sell ongoing membership" option
Since we don't store credit card info, reocurring billing info in the CMS how does the customer change payment type, frequency, cancel, and deal with expired credit cards? I assume this relationship would now be with the payment gateway vendor and not my BC website?
You can do all that from the orders screen. Some things can be done on the original order and for some things to need to stop the recurrence on the original order and create a new recurring order. Note that the charge is always initiated through BC. Also, customer can't do any of these things from the frontend. It is the site admin who needs to make these changes inside the admin console.
So, to address those individually:
change payment type - The payment type for recurring payments is always Credit Card. This can't be changed to any other payment type
frequency - frequency can be changed on the original order once it's created. The job that's ran every day looks at the frequency and creates an instance of the original order 14 days prior to the charge, then the instance is charged after 14 days.
cancel - Customer will need to contact the site to cancel the recurrence. The site admin needs to go to the original (master) order and set to recurrence to disabled. If there are instances of that order already raised those will be processed unless you set the Direct Debit to None. If you do so, all charges will be stopped from there on.
deal with expired credit cards - site admin will get notified of this and will have to contact the customer to obtain the new details and update CC details. Alternatively, they can send the customer to update CC details form somewhere on their site.
Note that the recurring orders can also be created manually through admin.
I hope that helps!