What really happened on the boat in Blackwater?

Solution 1:

To answer your question

are there any hints or evidence that he might already have had a hand in ensuring that the Blackwater events lead to disaster?

No

It is stated that Micah turned spy after the gang returned from the Island.

As far as an explanation for your main question

Are the details of the events intentionally left unclear for the entire game, or are there some actual accounts of what happened on the boat in Blackwater?

, We do not know any details about what went wrong in Blackwater. We know that it was a failed Ferry heist that resulted in several deaths of fellow gang members and the gang is trying to make enough money to retire for good. If you're wondering where my information is coming from, I've completed the game twice and have quite a good memory :)