Not sure if this question is still active. But yes, I think you can do all this.
1) You can configure which ldp is available for each client by setting IdentityProviderRestrictions on the client ( docs )
1.1). Not sure what you mean, I believe one of the idsrv availability points is to set up your authentication, and this makes it easy to integrate future websites with the same service.
2) When you log into the system using the client (application), you also indicate which apiResource the client has access to, and the application should add this to the requested areas at login. Therefore, if your client is an mvc application, you simply add ApiResource to AllowedScopes - and set request_type to id_token code - then this will give user a access_token , which is transmitted with each request to the backend api. ( docs )
2.1). This will basically register the user on both sites - using an access token that says that the user has the right to use backend-api.
2.2). In my opinion, this stream is one of the factors that make idsrv great - and they even mention it as a great feature of idsrv itself. To get access to all systems you just need 1 trip to authserver.
as for pt. 3 - Take a look at the documents, try installing an empty project after a quick start.
To enter the system from your own login page, you need to use the Resource Owner password grant type - if they do not recommend doing this for security problems (passwords through a wire) - it is supported.
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