OTWO,
You are being a good grandson trying to hunt down your uncle. I see why you are angry. Still, since you asked for opinions, I would definately try to track him down and let him know what is going on with his mother. I think your right about his sanity.
Its weird that he didn't leave contact information with is old congregation. They haven't forwarded his records, even though he was an elder and its been six months? Very strange.
There is only so much you can do and it looks like you've already done quite a bit. I doubt his old CO has any contact information since the old congregation hasn't sent his records over. If he's got a nonpublished number, its going to be even tougher to hunt him down. A PI could help, but personally I wouldn't go that far.
Sorry, I guess I'm not much help!