You could test the batt in another bike, right? You could also test shifters, etc but depends how much spare cable you have or if you have cable extender.
I believe there is a password on the bios too.
Without going into too much detail, it's an inherited ex corporate machine, I can log in and use it but can't make any changes/install anything
I'd either crack the password or try and get a copy of Windows to install from somewhere. You used to be able to get BIOS password crackers. Presume you still can although I also presume it's less easy now. I've not had to do that for 20 years or something.
Do you have more than one Di2 bike available?
You could test the batt in another bike, right? You could also test shifters, etc but depends how much spare cable you have or if you have cable extender.