[Arran]

I think that the best method of repairing the joints at the top is to convert whats left of the groove into a rabbet joint by chiseling what's left of it out. It's end grain so a sharp chisel can clean it up quite nicely. Even easier, if the rest of the cabinet is already coming apart, is to take the sides off and clean it up what's left with a table saw. I remember reading that the type of joint used was called a lock joint, sometimes called a locking rabbet, meaning that it can only come apart in two directions, a double lock joint can only come apart in one direction. They used to use things like this because of the crappy hide glues they had at the time, they needed the extra surface area for strength. They used to use another called a glue joint to join the edges of the two flat boards together, much for the same reasons, you see those used in old table tops.
Regards
Arran