Electrical – Low voltage under load from main line

These kinds of problems are very difficult to diagnose without being on site. Rather than guessing at a single fix, let me give you a process to follow. Think of electricity as a closed water system, with hot water coming in and cold water draining out. What we are looking for is a leak in the system. I'd start at the breaker. Check the load screw to the ground/neutral buss. 120vac ok? yes, then we move further down the line to the next possible junction point. At each subsequent point, check your hot side to neutral and to ground until you find the point where there is no longer a proper 120vac measurement. When you find this point, if you have 120 hot to ground, but not to the neutral, you have an open neutral. Turn OFF the power and check the neutral to ground with an ohm meter. It should be very close to 0 ohms. If you see no voltage to either neutral or ground, and the neutral to ground is 0 ohms, then you have an open hot. Check the input side of the GFIC and the load side in a similar method. If you try to follow a very straight logic, testing the three possibilites at each point,(open ground/open neutral/open hot) hopefully you will find the culprit. Good Luck

AFCIs are also a bit like GFCIs in that they care a lot about crossed neutrals. A LOT. You will never get any AFCI to hold while a crossed neutral persists, nor should you! Remember, neutrals do not have breakers, the only thing that keeps them from overloading is the fact that they are fed from only one hot. If they are fed by 3 hots, one could return 45A and of course start a fire in the walls.

You will have to identify every outlet (including hardwired ones) on each circuit. Pull the covers off and the devices out. Document how they are wired. It is likely you will uncover the problem at that point.

If not, disconnect them all and light up each circuit, one at a time, to determine which is the "first" outlet. Hook up that outlet correctly (as opposed to how it is now) and test. Continue one at a time, thoroughly testing each time.

When you run out of things to hook up, and you still don't have problems, then you have licked it. Otherwise, the problem will return at a particular step. That is your problem area.

You seem sanguine about some crossed circuits in a 3-way light circuit. That may be one of your problems, and there may not be a legit way to regain original functionality without going to smart switches.