Thank you Jolla for your not-answer. Thank you @sledges for trying to be indeed helpful by digging out some public available information, which was sadly mostly already known. The unbrick thing looks indeed promising, luckily I have a working J1 also so I can hopefully get the needed boot files.
What I can’t understand why not even a list of the connectors as seen in the older forensics article and their basic usage is provided. This stuff must be available, and can’t contain any information worth an NDA with a third party.