Patience will be your friend here.
Kroil is good penetrating oil, maybe the best, so you are already on your way with that.
Use some heat to help soften old goo (gas, oil, other) but don't get too greedy with the heat. Use a heat gun (not a hair dryer) to heat soak the entire area around the shaft in the carb body.
If you have access to an ultrasonic cleaner, you can try that with some safe solvents.
Heat the carb up, then toss it in the cleaner and let it sonic soak for about an hour.
Disconnect all linkages to the shaft being worked on to ensure you are only fighting the stuck shaft and nothing else.
One you have things heated up and soaked plenty then try moving the shaft forth and back.
Remove the nuts on the outer shaft and use a tool directly on the milled end of the shaft (be careful though).
With the shaft free from other parts of the carb linkage and plenty soaked and heated, if you are not able to s l o w l y get things to move open and closed, even just a little bit.
If this does not work then you can try coaxing the butterfly open with a wood dowl and a hammer... use gentle taps because it is easy to bend things and ruin them. But with careful force applied you can usually work even very corroded butterflies open eventually.
Again, be patient, let thing heat and soak, heat and soak, heat and soak, moving them from time to time to see if any further movement has been attained.
Eventually you will get things moving enough to fully open the butterflies, but this does not mean you have solved the problem just yet.
There can be a corroded/rusty/rough shaft inside the carb body and that stuff will wear the shaft hole pretty quickly if you try using the carb.
The shaft may be bent, or twisted, causing the binding.
The butterflies can be rusted, bent, misaligned too.
Keep at it, be persistent and patient and methodical and it will come loose..... eventually.
