Awesome write up and project. I just bought the same trailer a few weeks ago and finally (I think) have the wiring fixed up right (with the help of a dedicated ground wire) and am getting ready to build the stakes and side panels, and put the deck down.
One question I have for the OP: you mentioned that you used "3x8" x 1 1/2" lag bolts with nylock nuts" to fix the decking to the frame. I assume you mean carriage bolts or hex had bolts. I'm particularly interested in how you sunk the heads of whichever of these you chose into the decking so as not to make them pop up above the surface (which I assume would be a problem when loading certain types of materials).
I was going to try to go with a screw head that I could easily sink into the wood and make flush. On this page, e.g., they call it a "countersunk flat" type:
Menards sell some machine threaded screws like this and call them "stove bolts", but the problem is that they tend to only go up to 1/4" in size, not 3/8" like I really want.
Anyway, if you figured out a good way to make hex head or carriage bolts sink into the surface, I'd like to see how you did it.
One other question about the decking too, you say that you chose 3/4" CDX which is what the manual recommends, but I think I read somewhere that that is actually too think to really let the two halves fully fold up. Did you have any trouble with that?
I've also completed two other modifications that were somewhat inspired by what you'd mentioned here:
1) to hold the side panel stakes in, instead of using a cotterpin held bolt, I'm going to try using these cotterless pins. Since the hole they go through must be very close in size to the pin so that the ball bearing will properly secure it, I re-drilled 1/4" holes through all of the stake holders into the frame:
2) To hold the two halves together, I bolted one of these across the middle of the outside cross bars: