Just to follow up: this repair is done, and it came out great. The turnbuckle worked beautifully... I was able to dial in the right neck angle with much greater precision than I could with a clamp at the headstock. The UTB had failed only under the fingerboard between where the top had cracked. I was able to jimmy it open just enough where I had room to slide a piece of sandpaper under it to clear out the old glue, then I worked in some new hide glue and it's good as new. The popsicle brace was a pain to get out with the UTB in place but with patience and heat I prevailed and replaced it with a new one with more gluing surface area. I also added a spruce patch under the rosette and epoxied a couple of small blocks to the top and the heel block for extra insurance.
I think the best thing I got from the repair was a new tool. I took a hunk of steel and drilled it to where I could attach it to a cam clamp with a screw and nut. It easily swivels to be able to fit any angle which certainly comes in handy when you have to work quickly and blindly inside the box. It came in handy to clamp a heating blanket to the underside of the top to preheat things for gluing. I also used it to heat up the popsicle brace to loosen the old glue joint by heating the steel on a hotplate, then attaching to the clamp and positioning it inside the box. Certainly nothing revolutionary, but another helpful tool in the arsenal.
An old warrior gets a new lease on life:
