Success!
Removed the components Patrick suggested and it works like a champ.
The MMM switch is useless now, switching it off did nothing. So I bridged the contacts at the PCB (did this after the pic above).
And now, I can use the Castle Link without any mods, nor do I need batteries hooked up to the ESC. I don't know if this mod makes anything run longer or better, but I feel better with my V1.