This is all true Idaho. However 12kt gold is just not a common alloy, you will be spending a ton of cash getting custom alloy. Better to stick with 10/14/18, you could even go middle eastern with 21/22k. If someone brought me a necklace marked 12k I would first assume it was gold filled, if it tested on acid I would pay 10k price for it because they don't make a 12k acid test... You would have to find a very open minded gold buyer probably with an xray gun to buy that necklace from you at any reasonable percentage of spot...
Probably more reasonable would be to make Gold ingots in the shape of some sort of carabiner, then you don't have to cut the chain every time you want to take a link off or add a link.
Another option is something more like Chinese coins, get Rodebaugh to make you some miniature Rodorolos with a hole in the middle, call them rodolifesavers and put them on a string or a gold rope chain
Then you get 999 gold, easy to add/remove etc.