Simso I'm sure you have excellent reasons for going in through the back door, but in my opinion it will only make things harder.
Rocket, if you did that, you would have to support the bass. top down. for quite a while, that fingerboard is really going to get in the way
. But with the top off, you can throw the bass in the corner, do the top repair on your bench, and bring the rest out when its time to glue up again. Don't take the fingerboard off.
A double bass has very deep ribs and to do any useful work on the top you need to remove the top, as working any other way leaning into the instrument is really difficult. Plus, the back is usually glued on with stronger glue, and has a functional (and decorative) attachment to the neck at the button, even on cheap junky instruments. That is why i would do what I do on every instrument that comes in needing top repairs; remove the top. And yes, I only do double basses and cellos. You will need to remove the top to be able to remove all the bad repair wood, back to the original. Yes you will find yellow glue white glue and God knows what inside. Yes you will likely damage the ribs and the edges of the top when you remove it. But the top glues along a flat surface. So provided you can restore a solid gluing surface to the edges and the rib linings, its all good. (this is another reason for not removing the back; a back often has a bend at the upper bout and this makes it harder to get a good fit to reglue). I'd try a kitchen knife with a curved blade, dry, work it into the seam slowly and if you're lucky the seam will pop open. look for places where the seam may already be giving way ... good place to start the knife.
You'll need to work the knife, or knives, around the perimeter being carefully to only separate the joint, not cut into the top of the ribs. Heat may help. At each of the corners and at the top and bottom there's a block, but as its a BSO they probably won't be glued up all that well. But you will need to work the knife in carefully around the neck joint to get that area to release easily. take your time. You will damage the edges in places. But you just have to repair them again. Standard procedure.
I would repair the edges where the ply delaminates with good strong glue and any wood you care to use. Its only plywood. plane back to shape.
I would then remove all the bad repair wood on the plate with a chisel and a plane, and perhaps soak off the last .5mm. But you could just sand it out too. This is not a valuable instrument yet
Then I would reglue the top cracks, pressing between paper or foil and perspex to make sure the edges all line up nicely. You don't want to go shaping the outside if you can help it. Epoxy would be fine and fill any gaps nicely.
Then I would use say a 4mm piece of guitar top as a patch material and fit that to the inside of the top. You'll have to see how best to get a really good fit. Heat blanket and sandbags might do it. or you might need to chalk fit a bit. You can do it in two halves if that's easier. Go edge to edge between the FFs. Glue with whatever you like. I have used just radiata pine to patch a ply bass top before. It works fine. you do need to get a good fit though.
Then i would plane down the edges of the patch, leaving a nice flat bit where the soundpost will go.
I cant really tell from the pics but if the edge of the FF is misaligned this is most likely because the top is deformed. Much less likely someone has inserted flat ply in there (although i could be wrong) To get the top back into shape you need to press it back out to the right curve using heat and weights AFTER you reglue the crack but BEFORE you put in the patch.
Also check the ply at the back where the soundpost sits. if it is bulging you may wish, while the top is off, to add a veneer patch there too to spread the load. tis often done.
Hope this is useful.
There may be other repair methods, like pouring in bondo from the back, or falcate bracing, or magnets, or fibreglass, and these may work just fine, but it's not what I would do.
You have a pile of firewood there, you can rescue it with some time and care, and then you will have a cheap ply bass (about the cheapest money can buy) if the neck doesn't snap. Check the scroll chin and pegbox before you do any top repairs. If you have cracks there, yu'll need to fix them before its even worth having a go at the top. Is the fingerboard ebony? Hope so. Check by scraping at the cut end of the fingerboard with a chisel. If the paint comes off, its not ebony. Another thing you're going to have to deal with :-/
Have fun!
