I have a total rstoration on the bench..A 40's script Gibson...Neck re-set / refret bridge / bridge plate , etc....All the back braces need fixing or replacement...Now , the back has a full length crack as well as a bunch that were poorly " repaired " with glue residue mounded on each side of the cracks...Doesn't seem to be an organic glue...Cracks are wide open...I know it's a bear to re-glue a back once removed , especially on a bound back , but It seems to be the best way to repair / replace all the braces as well as getting at the bridge plate...My thinking is it would be far easier to just replace the back as opposed to trying to piece everything back and hoping I can get any of it to line up again , especially after fixing the cracks when the original back is " repaired "...Working through the soundhole is what every luthier loves doing , but.....BTW...Luckily , the sides and top are in good condition...
I would remove the back for full access and repair the existing back. It can be made :-)
Problem is , I doubt it will go back together well....Old glue in jagged cracks that have been open a long time...Curling up around the cracks as well...The biggest most open crack is the one that goes all the way across the back in nowhere near straight...It will never match back up again...
If the wood has shrunken, new wood have to be inserted somewhere. Maybe you can glue the cracks tight together with the braces off and split the bottom in the middle seam and add a wooden stick between the halves before gluing back the braces. Or add a wider binding around the edge, perhaps a wooden binding around the perimeter of the bottom with the old binding at the edge.
Hi, I had a Gibson parlor guitar that was split everywhere and repaired by the owner. In the photo you can see the patches I installed after cutting out the cracks. You can see what the owner repairs looked like as I left one as it was. The owner was deceased and this was a sentimental gesture for the widow. It was the only crack that was structurally sound.
Maybe something similar could be done on the back of your project.