Don't sell that bike.
I would say there is nothing wrong with that motor.
I had a horrible knocking sound from my engine that I was sure was a spun bearing. Turns out it was a bad coil!!!
You've got to much money invested to take a loss on it.
And your mechanic is obviously not good with GoldWings, he may be a good mechanic but he doesn't know GoldWings.
Before you give up on it get yourself a manual fromt he top of the Technical forum, buy a compression tester and set of carb synch gauges.
At the very least you can sell them with the bike if you have to.
Get those carbs synched before you go any further. The symptoms you describe scream out of synch carbs, at least to me. It wouldn't hurt to go over the ignition system too.
Check your plugs for any soot or "wetness" or any buildup. Then take a look at your igniters, they are under the fake tank on the left han side, a 10 mm bolt holds them both in place, pull the bolt and get a look at them if they look like this
then chances are you've also got an ignition problem. The potting material should be flush with the edges of the metal housing. Now being in that condition doesn't mean tat the igniters are bad, just that something else created to much resistence in the ignition sytem and they overheated. they either work or they don't since the bike will run they work. You can refill them with high temp silicon found at any auto parts store. They generate a good bt of heat on their own.
If the igniters do look cooked out get a good look at your plug wires and you coils. Check the connections at the plug boots and at the coils. Look the coils overs for any irregualrities in the surface, bulges these may be very slight, cracks if you have cracks you have a abd coil, or stress liines they look like white lines or scratces in he surface of the coil.
By the way if you'ld spun a bearing...let alone a couple, that motor would be locked up in short order. Or at least that's what one very knowledgable prson said to me when I mentioned it.
With any luck there will be a forum member nearby to you who could ride by and take a look at it for you.
Either way I'm beting it is something in either the carbs or the ignition. Neither of which is wrth taking such a loss on the bike.