Pete,
You indicate a certificate generated from IIS is ''known to fail'. Is this by design or a bug?
Re web adaptor & trusted CA issued cert - presumably you this cert could originate via IIS as opposed to ArcGIS Admin? Or again, are you forced to go through the admin interface? Futhermore, on a deployment scenario with multiple web servers (with web adaptors/load balanced), presumably one can share the cert with a similar common name across each? Or are you against forced to have unique certs from web server (with web adaptors)...
Hopefully i've explained myself clear enough...
Brad