I believe that your cables are not D-25 SCSI cables. Serial cables just won't work. You need the blue cable that Iomega originally provided or a similar one from another SCSI peripheral provider (CMS definitely sold one in the Plus/IIgs days).
The main clue is in point 1 where you say that the eject button works on the Zip drive. If the drive was being recognised correctly by the Mac, the eject button would be disabled (ejection is handled in software by the Mac).