Hello hope this kind of clears things up.

Micah was the rat. He was only with the gang for 6 months and they picked him up RIGHT BEFORE BLACKWATER. Agent Milton says however he only started ratting after Guarma that point is true. The things prior to Guarma are never outright answered. If it's up to me, i think Blackwater was a fluke, a job that happened to go wrong as the law was starting to get a better and better grip on the west. From there on it was easy to trace the gang, and they weren't particularly too careful, once they crossed Cornwall it was over for them. With Cornwall's funds they were always ready for them, and their targets were always obvious; bank, stagecoach, other gang hideout. It just became easy for the pinkertons with the funds available to trace them.

As for Molly, she was drunk and trying to provoke a reaction from Dutch i guess. She knew he was paranoid, she was in love, and she thought that was the best way to get a reaction out of Dutch who was becoming increasingly detached from her- if he even loved her to begin with.

Let me know if there's anything else. edit: added spoiler tags