I don't know if the OP is still looking for advice, but here's the perspective of another DM candidate:
400 yard swim test: After conducting a self test that would have earned me a 2, I paid for an hour-long session with a swim instructor. He gave me a solid diagnosis of my major form problems, along with worthwhile tips and drills. After the lesson, I practiced my stroke on my own pretty much any time I was in any water for any reason. My stroke got smoother and slower. Getting eyes-on from someone who knew what he was looking at made all the difference. He told me later he knew just from my body type what my problems would be even before I got in the water: muscular guys need to learn not to attack the water. The pros are worth the money. (For the record, he told me not to reach deep on my power stroke.) I just took my test for real last weekend and earned a 4.
Tired Diver Tow: I practiced different techniques with a buddy and decided to grip the tank valve with my left hand and kick for all I was worth in a side stroke on my right side. My instructor said the key is treating it like a sprint and not pacing yourself. I believed her and went all out. My legs were cramping at the end, but I got a 5 with a few seconds to spare.
Gear exchange: The keys are 1) establishing breathing routine and communication, 2) going slow. There's no time limit. Just do what you can without hurrying on each breath cycle. You have all day.
Best wishes,