Hide glue is the only glue to use and you don't need to remove the old glue from the joint before regluing. If any dirt has entered the joint that needs to be cleaned the best you can.
You don't want to run sandpaper through the joint and change the joint contact.
To clean the joint I'd first use compressed air and then clean with water and a toothbrush.
I have a bowl back mandolin that I disassembled for the rosewood (patch material) but I won't have time to take and post any pictures until tomorrow afternoon at the earliest.
In a nut shell. Think of it as extending the heel shape on into the body for about 1 to 2 inches then cut a rabbit into it so the bowl material will lay flat with the heel and you have the basic idea. There isn't really an "joint" as we usually think of a dove tail or a head block separate from the neck. Overall, the design makes a lighter joint than separate block and neck but it also means that any bad glue at the joint where the staves meets the neck practically guarantees that the instrument will start to fold up.
I will try to get some picture tomorrow if no one else comes up with some.