Here i'm explaining solution for manage multiple API Manager environments and external key management server to build central API repository. Gateways in each environment can talk to different key managers as its just pointing key validation URLs. In store we do not have concept of showing multiple consumer key/secret access tokens per environment.
We have one API which can deployed in multiple environments. But can only one subscription for all environments and same set of tokens will work any environment. Still each environment have their own key manager and when it comes to validate again direct to same external key manager.
To understand more about API publisher publishing to multiple environment please refer this article.
To study about deploying same API across multiple environments and URL to automatically resolve based on deployment refer this article.
External key manager concept and more details.
Display multiple gateway URLs in API store can achieve with external gateway concept.
Please refer following diagram for complete solution.