This is puzzling. So I presume that /var/hda/files/drives is owned by firstadminuser:users as well. If it is not, it should be.
This sounds like one of those hard to track down issues. Have you considered uninstalling DLNA app, cleaning up any left behind files, and reinstalling? Maybe there was something changed in the app since you installed it that will correct this problem.
Sorry I am not much help, but not sure what else to do at this point.
I don't think it's a DLNA issue...the issue is with the system itself because when those files are copied to the HDA remotely shouldn't they be assigned the correct ownership by the system? It's just a symptom that HDA isn't able ot access the files because they do not have the right ownership.
You are correct in that the parent directories all have the right ownership
/var/hda/files/
/var/hda/files/drives
/var/hda/files/drives/drive1
/var/hda/files/drives/drive2
/var/hda/files/drives/drive3
Directories created under those also have the right ownership when new files are copied and created
For example 2012-11-25 - Thanksgiving
var/hda/files/drives/drive1/photos/2012-11-25 - Thanksgiving will even have the correct ownership. It's only the files copied there that will not have the right ownership.