Progress! I found that the problem with the horizontal centering was not due to a bad control pot. The issue was with a two section capacitor that I mistook for a single section. I had been shorting out C207 to C208 which shorted the upper half of the wiper. I rebuilt the capacitor again with both 100uf caps in place and the horizontal centering now works as expected. This repair also allowed me to adjust the focus coil for its intended purpose of uniform focus instead of adjusting it to center the raster. Now I have a centered, fairly well focused image
.