Don't have any flux right now, but the flux-core solder worked well enough. Pre-tinned all the wires and didn't have any issues. I did three red wires as suggested and the point where all three attach is the ugliest part of the whole re-work

Everything seems to check out, but it's not working. The top two blue wires measure 4v and the bottom 2.5v

Can the wiring work the way it's illustrated above, with the constant voltage applied to the FET's drain instead of source? In other words, backwards from how the FET would normally work.