Shopify storefront for Sass aplication

[Xoshi]

Hello, 

I am researching possibilities to achieve such flow: 

1) Having landing page built via Shopify where end user can subscribe or buy digital products.

Domain example: (my-sass-name.com)

2) Having a sign-in page to be part of sass-web-app, user would enter their email, submit the form (it would execute endpoint to my own sass web app. 
Domain example: (my-sass-name.com/login)

3) Then, if user is subscribed or bought digital product, after they successfully logged (email, passwordless auth) in, they would be redirect to my sass web app.
Domain example 1 (my-sass-name.com/sass) (if possible I would like to do redirect under same domain, just different path name). 

Domain example 2 (app.my-sass-name.com) if example 1 wouldn't be possible to achieve. 



The main questions:
1) Is it possible to achieve something like this by building custom app, that would be available in app-store? 
2) Would it be against shopify T&C ? 


Any info would be appreciated. 


UPDATE:

SO my vision would be something like this - when user becomes subscriber, or buys a digital product. 
(There would be webhook that would notify sass-web app backend, so I could update user list). 
Or if that's not achievable - it would be possible to periodically fetch users to sync it with sass-web app db.