Degrafting Help Please

[Chief BigTittyFlapFlaps]

any tips for degrafting a scion that has grown around the stock?  any special tools/techniques?
   

[Backyard-Chief]

The stock of this plant became squishy and detached.  it was in a clay terra cotta with store bought cactus mix.  the outside of the scion is firm all around.  here's how i handled it in pictures...

1.  do you think this one will be ok?
2.  what would cause a stock to die out like this from the centre up?  strange.  the others have not had this issue.     

[Backyard-Chief]

...

[Backyard-Chief]

...

[Backyard-Chief]

...