We have you covered. All of the source code for Software Licensor is available on GitHub. The service is hosted serverlessly with AWS API Gateway, Lambda, and DynamoDB. The backend uses a custom cryptographic protocol with standard cryptographic primitives.
Multiple stores cannot sell licenses for the same product, although the client-side code could be adjusted to use multiple Store IDs.
Offline license activations and subscription licenses are not currently implemented. Not to worry, offline license activation and subscriptions are already implemented in the API's backend, but they would need to be implemented in the WordPress Plugin and in the client-side code (except for subscriptions; subscriptions only need to be implemented in the WordPress Plugin).
Subscriptions aren't fully implemented because WooCommerce Subscriptions requires a subscription. Offline license activations aren't implemented because they require a new form to show up on your website, and a new component in the C++ code. More information about offline license activations can be found here.