Don't be too hard on yourself. If you have been around the automotive industry we all have been in a situation like this. It comes with the territory. Look at the bright side. When you get this figured out, and I bet you will, you can be the "cruise control Guru."
My thoughts are that you are right. The service manual does not have all the answers. It can't. You wouldn't be able to lift it.
Some things just take experience to solve. Like what you are experiencing now. When I had issues like this I would usually do exactly what you are doing. Get frustrated and ready to quit. Then it would bother me so much I would go at it again. Lots of frustration but a good reward when you figure it out. It's extra hard for you with little equipment.
Some of my oddball issues turned out to be bad connections. When you test with a meter minimal amounts of current flow through the conductor. You get normal results. When you try to use the cruise much more current is present and the high resistance drops voltage to a level that it will not function. You seem pretty smart and I am probably not telling you anything new but sometimes you need to test the circuit with a load.
Pick something very much at random. Say....the cruise set button and circuit. There is a
bad wire. Maybe just a single strand holding on by a thread. Could be a switch that has poor contacts. When you test it with your meter 1 /1000 th's of an amp flow through the circuit and all checks out well, as it is able to handle that current. Now when you set the cruise 1/2 amp flows through the circuit and no joy.
In a case like that a test light might show the issue as it supplies a greater load. It would be like "I have 12 volts with my meter but the light won't come on." This might be the issue with cruise set, front brake, rear brake,throttle switch etc. Might even be an intermittent problem. What if the ground for 4th and 5th gear is poor or jumps around when you go down the road. You might have to "mock up" some of those things to find the issue. That is ground the 4th, 5th gear by jumper wire. Send 12 volts to the appropriate wire in the module directly from a "hot" source. I would purchase the module as a last ditch effort only after I was completely sure everything else is fine.
To add to the difficulty is trying to communicate via this crazy machine. The guys here are good guys and willing to help but it is much more difficult to explain things some times that it is to actually do them. I think through this whole ordeal you have come to recognize the logic of the thing. That is it has input (set etc) safety, and output signals. With that knowledge, good information and persistence you can do this. Then make sure you check back anytime someone has an issue with cruise control. You could be the best.