7.6.3 firmware definitely has more bugs than previously.
IMHO the first step is to downgrade the TC.. use 7.6.1 or even 7.5.2 on an 18month old TC might work better than any 7.6 version.
Secondly 6.3 is problematic as a utility.. apple seem to have made ipv6 local-link a necessity..
See the rather belated advice..
Downgrade firmware is easy in v6 utility.
Simply select version with option key held down. Select the version you want to use and it will downgrade.. (or maybe that is an upgrade!!)
You can also use 5.6 utility to manage the TC.. since it is not the latest greatest which needs the latest not so greatest utility.
How to load 5.6 into ML.
1. Download 5.6 for Lion.
Click to open the dmg but do not attempt to install the pkg.. it won't work anyway.
2. Download and install unpkg.
Run unpkg on the desktop.. it is very simple.. drag the AirPortUtility56.pkg file over to unpkg.. and it will create a new directory of the same name on the desktop.. drill down.. applications utilities .. there lo and behold is Airport utility 5.6 .. drag it to your main utilities directory or just run it from current location.
You cannot uninstall version 6 (now 6.3 if you updated) so don't try.. and you cannot or should not run them both at the same time.. so just ignore the toyland version.. the plastic hammer.. and start using 5.6.. a real tool.
For screen shots see this post.
You only need to do this on the computer that is going to manage the files.. but it is easy once you have the actual 5.6 utility to just copy it to every computer in the network.
Sounds like interesting stuff.
The TC is not designed as a NAS.. so if this really doesn't work out, I would be looking at a 2 disk synology so you can run mirrored raid.
And of course backup is so much easier in a real NAS.. TC has no inbuilt backup ability, and TM cannot backup network drives.. so Apple kind of didn't intend TC to do this job.