OK, so I have installed the T2 certs on the T1, and added an extra one of my own to confirm it is using the new certs. My test cert works!
installer: PRS-T2_cacerts_for_PRS-T1.zip (117.35 KB)
https://workupload.com/file/jEAeKJG
But… still the T1 will not download from OverDrive.
I wonder what part of the OverDrive download process is filling on T1?