John,
You are correct. The large L brackets on the idler car keep the boom in place. They are wider than the boom to allow for play. I used the same set up on my model and it works just fine, just make sure that there is lots of slack. The space between the L brackets should be 1.25 to 1.5 times the width of the boom. With the boom free to pivot this works just fine.
The two iron straps that you refer to are cable guides. They are attached solid to the boom but not to the A frame. It is hard to tell from the pictures but I do have that in my notes from when I examined OP first hand. In Sloan’s book he claims that the boom could move up to 60 degrees either way but required blocking and/or guy lines to deadman anchors to prevent tipping at all but the smallest angles.
Sloan also claims that the derrick winches were run by steam supplied from the turret of the loco supplying the power. I also noted that the glad hand on the back appears to be of standard air type, and it is possible that during one of the rebuilds the engine was changed over to run on compressed air.
As for the ditcher, that is really nice. I’ll bet it isn’t cheap though. Would make a heck of a winter project.
Ron