Submitting app for "beta usage" - outside of our own company domain

We are trying to submit our application - as a first step to be accepted to allow external company domain names to be able to use our app (aka the beta stage). However, running into some questions :
(a) when we have an app which is extended beyond our account, the app still allows a single callback URL. We can create a canonical URL (like an API gateway) - but will zoom pass us anything w.r.t the zoom account which submits a call to the app - how will we know which domain/zoom account sent a request so we can route the request from this neutral (multi-tenanted) URL to some customer account specific processing and
(b) we are currently undergoing a security review with Zoom marketplace security, once our app is approved say for the external account beta, what are the steps needed for full permission for marketplace - e.g. it seems like the domain ownership step is needed ONLY to publish to marketplace but we are not sure.
Any help appreciated