I made sure 1000 times I only told it to wipe the External.
DBAN does not have USB drivers, so you had two choices:

(0, 0, 0, -, -) Hard Disk
(0, 0, 1, -, -) Partition

Both of these represented the internal disk.

USB disks never appear and are never selectable. (This feature might be in a future release, though.)

The PC drive was labeled and the external wasn't, so I couldn't get mixed up.
DBAN does not read filesystem labels.

If my data is indeed gone, regardless of how or why, is there any possibility of recovery?
No, there is no recovery.

If not, is there any possible way to retrieve the list of files that were on the drive before it was wiped?