Nice one, it’s strange that you had 12 volts at the ignition switch on C0099-6 which is fed by fuse 53 and fuse 53 was live both sides but then as you say it’s odd that you lost voltage to 9-6 when you turned the ignition. I’m wondering why the wire between fuse 53 and 9-6 has died.
If you are happy then I’m happy , apologies if I came across as being short, I know how confusing these vehicles can be, especially when it comes to the electrics.
When you were checking voltages were you taking your Earth from the chassis somewhere ?