Mike,
Your sitting is actually your most stable position, in fact I regularly suggest the sitting for sighting in a gun at the 50. Based off what you wrote about your other positions I can offer a couple thoughts. Your group might move a little left/right as you move back to the 50, that is OK, get the windage dialed at the 50 and everything else will be good to go.
Your 25 barracade - sounds like you are putting way too much pressure on the barracade. If I push too hard I push left with the right hand and right with the left. Ease up on the barracde. If it is only left handed and the shots are going left it is one of two things. You are getting too much finger on the trigger and pulling the gun to the left or you aren't on the front sight hard enough and letting it drift. After re-reading your description, it sounds like a combo of both.
Also, with your sitting position, first thing - WORK BOOTS. If you try the sitting in tennis shoes you are almost guaranteed to roll onto your back a good pair of boots will give you counter balance. Next, let your arms hold the weight of your upper body not your abdominal muscles. This will allow your gut to relax and you can breathe for every shot. It is OK that the gun is in contact with your leg, just make sure if is against muscle not bone. The muscle will absorb some of the recoil and help hold the gun while bone will act as a pivot point and it will hurt. Get your sitting position under control and you'll be putting 6 X's down range in no time (it does impress when you can put six rounds under a fifty cent piece from the 50).
For the prone you have to be careful to make sure the grip frame is not in contact with the ground. That is against the rules and that contact will throw your shots off. I have tp ask if you are using prescription glasses (should have asked earlier too)? If you are, make sure the front sight is centered in the sweet spot of your focal point. If it gets out of the sweet spot your shots will drift due to the refraction of the prescription. For me, if I don't get my head just right I find the sight in the upper left of my lens. If I shoot with the sight there my group goes low left. Also, many shooters have to find a secondary aim point for the prone position, myself included. For me, my aim point moves from the neck/shoulder junction to the neck/chin for prone.
It sounds like you have the basics down, now you need to refine your technique. I will try to remember to bring the camera next weekend to get a couple shots of the positions and how to set them up. It would be much easier in person though.
Follow through is always an issue. My technique for it, especially in the longer stages of fire, is to hold the trigger to the rear until the gun comes down from recoil. Basically breathe, squeeze, boom (hold trigger back), recoil, sights come back in line with the target, release trigger and repeat. This will also help with your timing allowing you to use almost every second of your 165 for the 50. I usually finish with about 2 to 5 seconds to spare.
Make use of the dry fire targets too.