Just as an update:

I never did get the control plug off. I ordered a case of electronics cleaner to try and get the grime off the plug but still couldn't get it clean enough to get a good grasp on it. The angle from underneath is very difficult and I can't seem to ever get two hands on it.

I did however replace the speed sensor. The angle of the sensor is much easier to get to than the control plug. My 1" socket was not deep enough and threatened to slip off, so I ended up using an adjustable at a weird angle to get more surface area on the plastic nut. It worked. I did get a small amount of fluid leaking, about a tablespoon. I put the new sensor in and tightened using the same weird angle with an adjustable. I couldn't get it as tight as I wanted due to the angle, but it's plastic and it doesn't need to be cranked down anyway.

Long story short, the speed sensor replacement appears to have solved my issue. It shifts out of 1st and keeps the engine RPMs at a reasonable level most of the time once again .