Here are a few pics for reference. The first is of my stem. The second is the E-tiller.
Since taking the first shot I spent several hours with various tools (chisels, plyers, vacuums, hammers, Dremel, screwdrivers) and I have it down about half the depth it needs to be to properly fit the emergency tiller. In a true emergency it now has a chance of working, but I would like to get it deeper if I can. My tools are hitting length limits to get down in so far in such a narrow tube.
On a related note, I tried to slide the bolt out of the way but the larger cavity this is all in only allows the bolt about 3/4 of the way out. I suspect I would have to get under the deck and disconnect from the quadrant or the rudder itself to pull everything up on deck in order to remove the bolt. That's not a project I'm willing to do at the moment.