Alrighty, so the harness are all fabbed, plugged in, and everything works super. Pleasantly surprised to see that either there's no momentary loss of ground when switching between beams, or the ballasts hold a charge longer than that interval. No dipping between highs and lows.
Almost.
There's a minor issue with the current harness that causes a pretty undesirable side effect...
Fortunately it operates perfectly safely and I can use it until I install the fix as posted below. I need to do a resistance check tomorrow to see if I am going to need a resistor to make it work, but it did turn out simpler than I ever thought it would need to be.