As an update for those who are interested:
We've had some success. After a few more tries with Linux, we were finally able to convince the drive to mount for a while. It showed some confusing data at first (claiming there was a lot more stuff on the HD than there really was, so that may confirm my virus suspicion), and was still futzy, but it was at least reading a little.
We tried doing a formatted partition, and after some confusing errors (it refused to format), it started showing the correct amount of data on the drive. Thus, some hope that maybe it wasn't too far gone! Still lots of disconnection errors and failures to mount or access, however.
We were pretty sure at that point that part of the problem was damage to the connectors or physical damage of some form, so we tried unboxing the drive to connect it directly into the PC. Alas, it's ATA while the system is SATA (or the other way around... my partner's been the one handling most of this, as I've been at work for most of these events and just receiving updates, and I forget which.)
Well, we had an old SATA-ATA converter cable lying around, so we tried that. Aaand... it didn't fit. After checking the various plugs and bits, we determined that the connector on the drive is a proprietary one, and now we've actually contacted Western Digital for further information and maybe replacement parts.
Reassuringly, the response we got after a thorough description of our exploits was "I have directly forwarded your problems to our support team." We're waiting for further word now.