I have a elderly friend who has an older KM-675. Good sounding little mandolin, but it had a broken neck when he bought it. It had been "repaired", but the joint was visible and palpable. The crack is below the 1st and 2nd fret on the backside of the neck. It follows the grain. I repaired it again for him several years ago and it held fine until he dropped it recently and the peghead broke off. I repaired it again but there was not enough wood on each side of the truss rod pocket to have enough gluing surface for a solid repair. It has separated again, as I suspected it might. My next approach might be to facet the back of the neck and add some maple splice plates overlapping the site of the original separation. Then I'd re-profile the neck and hide the evidence with a dark sunburst. The problem is there is so very little surface to work with. I've also considered drilling and interposing a couple of carbon fiber rods through the separation. Not a good idea, I don't think.
The right way would be to completely re-neck the mandolin. The problem with that is getting the polyester finish off at the neck joint and being able to refinish it without it looking crappy. And it may be difficult to extract the neck without damaging the neck block.
I don't charge him for any work I do, since he has little money and has been a friend for 30-some years. I still want to do it right and I hate to see him trash the mandolin, since it's better than his old Alvarez.
ANy suggestions would be appreciated.