I found the answer to Question 1. It's possible to retrieve an AppSubscription through these graphql queries: https://shopify.dev/apps/billing/reporting#query-for-an-appsubscription For some reason a was expecting to find what I wanted from this page...

Users using my app should be able to choose between a monthly and an annual plan. I understand that in order to create a recurring annual charge I need to use the graphql api https://shopify.dev/api/admin-graphql/2022-07/mutations/appsubscriptioncrea...