"It seems counter-intuitive that raking the mast back would reduce weather helm since the center of effort would seem to be going in the wrong direction, but that's what happened. "
Something’s not quite right here, because too much weather helm
NEVER IMPROVES with more rake. Not having a positive lock down pin/bolt through the rudder cheeks and head just isn't a good way to sail the boat with repeatability and consistency. You say that you have about 1/16" of slop at the pin, but that's at the radius from the pivot bolt to the pin. If you take the distance from the bottom trailing edge of the rudder and divide that by the distance from the pivot bolt to the pin, them multiply by 1/16" (.0625) , you may be surprised at how far out of vertical the trailing edge can go!!!!!!!!!!!!!!!!
The flip side is that without a positive lock down, the rudder blade can and will float upward/forward in light air, making for a really bad feel, squirrely steering and terrible pointing!!!
And before I get the nay sayers going here; although the 7.9 rudder blade is heavy as the dickens, it floats like a cork in water. So yes my friends, "can and will float upward/forward in light air" is a reality, and I've had it happen before, many moons in the past.
Something else to consider is that the pivot hole in the rudder cheeks, pivot hole in the rudder blade head and the stop pin that's supposed to keep the blade in the "right spot" are all slightly different from boat to boat and not exact to begin with. Do your self a huge favor and get the trailing edge of the rudder exactly parallel with the trailing edge of the keel (vertical to the water's surface) and install a real, foolproof lock down device. You will be amazed at how the boat steers and goes upwind afterwards!!!
I'm not saying that's your only possible problem, just the most probable. Bad mast tuning and an outhaul that's not tight enough for upwind sailing can create lots of turning moment on the leech. This can and will make the main want to act more like a weather vane than a sail. When this happens the boat (any monohull) will want to do the same thing as the weather vane on the barn........................... point straight into the wind!
