Hey all- wanted to wrap up this thread and give a conclusion. Mainly I need to give a huge, GIGANTIC shout out to JPBoyt for helping me through this process. I am a novice with electrics, and after getting a hold of him via this thread, he took time to diagram out what I needed to do, identified where I was misunderstanding things (like, having the WRONG SSR), and he basically gave me an espresso electrics 101 course via email and the phone. I am so grateful for his time and his expertise. I know many of you have used his services for controller repairs, and I wouldn't hesitate a second to send him anything I was having trouble with. I was just so impressed with the depth of his knowledge and his generosity with that knowledge.
To get to what we had to do: really, it was simply about understanding what was needed to trigger the AC SSR, where that needed to come from, and what were the various ways to get there. I ended up wiring the input side of the SSR with a "hot" wire from the pstat and a neutral wire directly from the switch. Because I ended up essentially adding 2 extra wires from the switch (a hot going to the heating element, and the aforementioned neutral going to the input side of the relay), I used piggyback terminals at the power light which is jumped directly from the switch. This provided a clean way to wire everything up and keep it fairly tidy inside. I ended up NOT needing a hi-limit because of the funky way in which the controller box is set up. Basically, the heating element won't turn on unless the water level is correct, so there is no possibility of burning out the heating element.
With the new heating element and the new wiring, the machine runs great and actually heats up faster than it used to!
As a side note, in speaking to Pat (JPBoyt) I discovered the burrs that he sells on his website. As I was in the market for replacement burrs for my MDX, I ordered them. I have ordered the standard NS burrs before, and they were fine, but WOW, these tool steel burrs were on a different level. They really didn't take any time to break in because they are so well machined. I know this isn't the spot to "plug" a business, but just wanted to put it out there. Not only is Pat a generous soul with his knowledge, but he sells some really beautiful burrs!
Again, thanks to everyone who chimed into this thread, and especially to Pat!