Because the gfci only trips when the old barn circuit is feeding it and not when the original circuit is feeding it, then you have a problem beyond a downstream neutral-to-ground short. I can't check your wiring from here and can only rely on what you've posted so far, yet you have not posted any voltage readings - causing me to believe that you are using the guess method to troubleshoot. You said you double & triple-checked the connections by seeing if the wire colors under the wirenuts matched up. This only eliminates conditions which would cause a breaker to trip, which is not your problem. You need to buy and learn to use a meter if you're going to do electrical work. The problem is in the wiring on the 2nd floor where you previously remodeled. This is what I'd look for if I were there: 1) in the box where you are attaching the old barn circuit, the original circuit branched in 2 or more directions. (Let's call this the branching box.) 2) you are not replacing the original circuit in the branching box, but rather are attempting to divide the outgoing cables between the 2 incoming circuits. 3) downstream of the gfci, 2 of the neutrals which left the branching box are tied together in the remodeled area. When you remodeled, you should have had one downstream box where the power coming in did not go back out on another cable. I'm betting that didn't happen, and you have a loop back to the branching box. 4) the 2 circuits are on different 120v legs, causing the gfci to see original neutral current which has reversed polarity from the barn neutral. It's also possible that the hots are tied together downstream at the point where the neutrals are tied. In that case I would have expected you to already have lit yourself up, yet that hasn't happened. So maybe you daisy-chained the hots through the receptacles rather than pigtailing and have re-used an old receptacle with a broken tab between the hots. Or maybe you have been turning off both breakers when you worked on the circuit, and the tripped gfci has kept the breakers from seeing a 240v dead short. These are just guesses based on the symptoms and results you describe, and perhaps they're all wrong. But they are what I would look for first if I were there. Best bet: call in a professional electrician. Your inability to find and fix this gfci problem casts major doubts on the safety of the pool wiring you're planning out of your new barn panel...

Dunno if this thread is still alive or not, but a little bit of methodical troubleshooting might help to narrow down the possibilities. First, can you put a cheap circuit tester (or an expensive one for that matter) to determine for sure that the GFCI outlet is getting power? If wired correctly, the test light should go on (or the voltage meter should register) when you test the two "line" terminal screws. If there's no power to the outlet, we can work backwards towards the power panel. [It might not be a bad idea to test the hot & ground combination as suggested above. A GFCI outlet will usually work fine without a ground. In fact, they are sometimes installed intentionally in ungrounded circuits under certain circumstances. This test might, however, point toward the diagnosis of a bad neutral.] Also test for power at the two load terminals. If there's power at the load and no power from the line, something is miswired, and we can go from there. If the receptacle is getting power, a next step might be to disconnect, temporarily, any wires attached on the "load" side (i.e., the feed to other outlets downstream). TURN THE BREAKER OFF WHILE YOU PULL OUT THE RECEPTACLE AND UNHOOK THE WIRES, RIGHT? If the GFCI outlet resets and works when no load is connected, the problem is probably a ground fault downstream. If, however, the GFCI still won't reset and work, replace it. Else, come back with test results. Cheers and good luck.

Get a different electrician. Something is wrong with the new setup, or there exists a return path other than the return wire. It is true that kitchen, bath, and outside circuits should have GFI circuit breakers, but I don't think they have to be on the same circuit! Each circuit needs protection by GFI, but each circuit may have its own GFI. Doubling up of outlets (not circuits) served by one GFI was done in the past as a cost saving measure. In my house, one outside outlet and the powder room is served by the GFI in the powder room, however, this is only one circuit. My house was wired in 1976. But care must be exercised in the layout to not create potential overloading of any one circuit. I expect codes have changed since then. At that date, GFIs were a new item. The general rule was that GFIs were required in locations where shock hazards were most common. Most of the locations were where one might touch a grounded item and a damaged or mis-wired appliance at the same time. Examples are: an electric fry pan and a metal kitchen sink or faucet; electric shaver and bathroom faucet; a radio dropped in the bath tub; and outside, a faulted device with the user making circuit to earth or a piece of plumbing; etc. Under these rules, places such as living rooms and bedrooms were exempt. However, local authorities may add additional requirements. The instructions for GFIs often state that these should not be used in series. For example, there could be a GFI circuit breaker in the main entrance box that is serving a bathroom circuit, but another small GFI was installed inadvertently in an outlet box in the bathroom in the same circuit. This would be two GFIs in series. If you are located in an incorporated municipality, very likely there is a building inspector and a permit is required to make major wiring installations. If this is the case, your building inspector can be an Allie in getting this situation corrected. In regard to the buzzing sound, have you determined the source? Is it a transformer in an LED fixture that is part of the power supply for the light? Transformers do present design problems and are prone to buzzing. Cheaply or poorly made transformers can be noisy. If this is the source of your noise, the solution is to replace the transformers with better ones, or get better fixtures.

but in general for regular fixtures, the power line goes to the fixture, then the black (hot) wire goes down to the switch and back. That's called a switch loop and while it is common, I don't know if it's more common than having power go to the switch and then on to the fixture. It all depends on the specific layout of the house in question as to which way is better (ie. uses less wire) And while a switch loop will have power still present in the fixture's box, it isn't actually one of the wires that is attached to the fixture itself. Those wires are still just the neutral and the switched hot coming back from the switch. The constant hot is wirenuted (is that a term?) to the wire going to the switch and tucked into the back of the box. No need to touch that if you are replacing the fixture. But as we've all stated, you should still turn off the breaker.