Dropbox Xamarin OAuth Flow Question - Retrieving the Access Token

As long as you're able to use it in Xamarin, using the Dropbox Java SDK is probably a better route. In that case, I highly recommend just using the pre-built Android app authorization flow. 

In that case, there should just be a few things you need to do:

1) Set up your manigest as shown here:

https://github.com/dropbox/dropbox-sdk-java/blob/master/examples/android/src/main/AndroidManifest.xml#L33

2) Start the authorization flow using startOAuth2Authentication as shown here:

https://github.com/dropbox/dropbox-sdk-java/blob/master/examples/android/src/main/java/com/dropbox/core/examples/android/UserActivity.java#L36

3) Receive the result using getOAuth2Token as shown here:

https://github.com/dropbox/dropbox-sdk-java/blob/master/examples/android/src/main/java/com/dropbox/core/examples/android/DropboxActivity.java#L22

Using that, you don't need to configure the redirect URI, etc. yourself. I'm not sure I follow your question about implementing Auth/AuthActivity though. Those are part of the SDK, so you shouldn't need to implement them yourself. If you're not getting anything back from QueryIntentActivities though, that should indicate that you don't have your URL scheme registered correctly. Double check your manifest against the link in #1 above.

And that's correct, development status is fine for now. Production status would just enable more users to connect to your app, but it's not necessary while you're developing and using your own account.

I'm afraid I cant offer help setting up the ImageGallery activity itself.