Without an O&M manual I would be inclined to get the UN arms inspection team involved ??.
When I started designing/ installation of various ESS systems at home, my wife asked if I died would anyone be able to maintain the system, to which I replied NO.
This then spurred me to compiling an O&M manual with schematics/parts ect and to be honest it's a god send - opening up a control box of multiple relays / thermostats / control cabling ect yrs after installing without a schematic is a head ache.
Would advise any DIY installers to at least do a drawing of the as installed installation
Good luck in the de- comissioning.