Hi everyone - just to chime in, after a week of dealing with this absolute nightmare, I'm finally able to get back into iCloud thanks to the certificate download listed above from Scrumper (I'm on 10.13.1, late 2014 MBP). I just want to thank all of you for being so helpful and communicative. I also want to thank dysamoria for starting this thread and teaching me about how keychains work and about creating backup OS installs. You all rock.