A sneaky way to get 12V to the coil would be to run a wire from the terminal on the side of the starter solenoid that energizes the solenoid. That way you only have 12V to coil when engaging the starter. In the schematic, I think I'm seeing a wire running from part #61 to the input side of the coil. Not sure what exactly that is, since Case/IH took away the description since they don't handle that part anymore. I have sent emails to the sites guru's telling them it would have been so much simpler to put N/A in the description, rather than eliminate the part description/number itself. It apparently fell on deaf ears, and why I have purchased several hard copies of parts manuals for some tractors I have. A necessity if looking for used parts, whether at salvage yards, or places like ebay.
You sort of lost me where you're picking up the 2V. Is the on the output side of the old ballast resistor, or at the terminal on the side of the distributor..?? If you removed the wires from the ballast resistor and jumped it completely, you should show 12V going into the coil. Now you need to check voltage going into the distributor. If it's still showing 12V the coil does not have an internal resistor. If it's showing 6V, it does. If it in fact does show 6V, then once the starter is released, and current is running through a functioning ballast resistor with output of 6V may be reduced even further going through a coil with a built-in resistor I'm thinking would reduce voltage even further, but not sure.
With the distributor cap off, do note the direction of rotation of the rotor button to get the proper firing order of 1-3-4-2. Not all IH distributors turn the same direction, but it should tell you in your manual.
It's probably a good thing you were only getting 2V with firing order out 180º, or you'd have more than likely gotten a big "KER-POW" firing on the intake stroke. Don't ask how I know... Let's just say it was a lesson learned many moons ago, and something you don't forget. But they will also do that with a cracked cap, or having a carbon trace between lugs inside the distributor cap.
If the new ballast resistor does solve those issues, might be an idea to get a spare, and put it where you know where it is. They have a tendency to burn out at any time with no rhyme or reason. I had one burn out of my old Case 310B backhoe that someone had converted to 12V. I put a new one on it but thought if it was going to be a problem, I'd better get a spare. With my luck at times, if it does burn out, it's 5 minutes after the parts store closes and I really need it at that moment.
It does sound like you're making progress, just stick with it. All the diagnostics you're doing will come in handy in the future if something happens. For a gas engine to run it boils down to 3 basic items. Air, spark, and fuel, in proper amounts and timing. Check the easy stuff first, usually spark and fuel.