From what I see a "killswitch" is nothing more than a switch across Hot and Ground which shorts signal to ground when activated.
If so, you don´t *really* need a full DPDT 6 terminal switch which I guess is shown on schematic to make it truly "universal" , as ion "it covers ALL possibilities"
Simplest option would be a N.O. spst switch: inexpensive-simple-easy to find in many sizes including 1/4" - 6mm
Normally does nothing, when pressed mutes/kills sound (if I understood your explanation).
Next will be an SPDT momentary one; plus an SPDT mini toggle switch which routes signal hot to either NO or NC contact.
You can select either always ON which you kill with the pushbutton or always OFF which you unmute by pushing.
SPST toggle switches are widely available for use in Guitars, and reducing pushbutton to SPST complexity will make your search easier.
In fact I remember mini toggle switches being offered in pushbutton versions, same body and contacts, so they esthetically match each other.
the full Monty: