With your old install, I belive you were very close to success. Because you obviously was able to browse the OS3.9 CD, and maybe it was possible to copy files from it as well. Then you could have copied the CacheCDFS file from "Emergency-Boot/L" to the L dir on your boot partition, and then copy CD0 from "Emergency-Boot/Storage/DOSDrivers" to Storage/DOSDrivers as well (making sure that it didn't overwrite any existing file). Now it only would have been a matter of editing the CDO file, where the Unit and Device lines got their correct values. Since you were using atapi_pnp300, I will guess either cd.device or cd_a600.device for the latter.
Then you could have used this CD0 file for mounting your CD-ROM drive instead.
With your new install, the main problem is that you cannot use scsi.device as Device in the CD0 file. This is because scsi.device in KS3.1 and lower doesn't support CD-ROM drives (which is what stuff like atapi_pnp300 fixes). Another thing is that the standard CDFileSystem only supports ISO9660, which in turn means that file names will be shortened to eight characters.