HH, this is one of those film puzzles that just gives us food for speculation. Fredo's involvement could have been as simple as telling J.O. which room was Mike's, or opening the drapes, or giving them some other insider info, but we're not privy to it. The only thing we know is that Fredo told Mike that Johnny told him Mike was being tough on the negotiations and if he could help out. Michael didn't even ask Fredo for specifics.

And how did those hitmen from NY get on the property. After many viewings and to complete a piece of the puzzle, I "supposed" they came in the boat that we see slowly cutting across the lake in the night. If you remember the scene, it is after dark at Anthony's party. There is a shot from the water, looking on shore at the lights, and a silouette of a small motorboat is seen coming from left to right....those were the hitmen!!! At least that's the scenario I came up with.