Authentication for Google API

Question

I'm trying to understand the flow how to authenticate user on WEB client (JS), and then use Google API on my back-end server (ASP.NET MVC application), on behalf of authenticated user for retrieving users contacts list.

Here the current flow that I use:

1.In HTML I use google JS client: https://apis.google.com/js/client.js:

function auth(callback) {
        var config = {
          'client_id': '***********',
          'scope': 'https://www.googleapis.com/auth/contacts.readonly'          
        };
        config.immediate = true;
        gapi.auth.authorize(config, function (authResult) {
            if (authResult && !authResult.error) {
                callback();
            }
            else {
                config.immediate = false;
                gapi.auth.authorize(config, function (response) {
                    //Here I send access_token to back-end using HTTPS
                });
            }
        });
      }

2.Then I use gapi.auth.getToken() and send it to back-end server (Using a HTTPS AJAX call)
3.Then on server I have the following code in controller:

public JsonResult Get(TokenModel model)
        {
            //Custom store for access_token
            var myStore = new MyStore(NewtonsoftJsonSerializer.Instance.Serialize(new TokenResponse() { Issued = DateTime.Now, ExpiresInSeconds = 3600, TokenType = "Bearer", AccessToken = model.access_token }));

            string[] Scopes = { PeopleService.Scope.ContactsReadonly };
            ClientSecrets secrets = new ClientSecrets() { ClientId = "******", ClientSecret = "******" };
            UserCredential credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
                secrets,
                Scopes,
                "user",
                CancellationToken.None,
                myStore 
                ).Result;

            var service = new PeopleService(new BaseClientService.Initializer()
            {
                HttpClientInitializer = credential,
                ApplicationName = ApplicationName,
            });

           List<string> result = GetPeople(service, null);

            return Json(result);
        }

Questions:

1. Is it the correct flow and does GoogleWebAuthorizationBroker is a correct class to use on server in my case?
2. Why and HOW GoogleWebAuthorizationBroker opens a new browser window for authentication, in case model.access_token = null?
3. Why when the token is not valid (ex: “dasdasdasdas”), AuthorizeAsync method returns me the UserCredential that looks absolutely valid, but then the exception occurs when make actual request to google api.
4. How from the above flow, I can get “refresh token” for later use (as I understand, I need somehow generate it myself, using access_token + secret key).

Thanks!