Query to get the next identity?

this one helps. Is there a particular reason you're using two pools for two providers as opposed to just using one? Cognito limits you to one application per provider per pool, but you can use different providers in the same pool, which would allow you to use the standard roles.
Edit: It sounds like the logins map contents might hold the issue. Future calls with an authenticated id have to have the same login token that was linked to it. So, if you created an identity id with Twitter auth, attempting to use that id without that Twitter token will give you that exception. Be sure you're including it/properly resuming app session on app restart...etc. I'd give that a second look, if you're not that would definitely cause that issue.

hop of those help? A Global Secondary Index can be setup on article Id & timestamp and queries performed against the created index for better read performance.

should help you out Anchor peers must be part of the channel they are the anchor for, as well as the endorsing peers.
Based on your description, of your 6 peers, you are joining only 3. It's not clear whether you are only joining the endorsing or the anchor peers, but if you do not join the anchor peers, it's possible you would observe this behavior.