I think you will find plenty of baby coddling required between TC and Lion.. but anyway.. you cannot easily use it connected by WAN.. I see no advantage to that, and just bridge the TC.. that will turn off dhcp (big no no in your LAN structure).. the TC will get an IP from the main router and function happily as backup device for the Mac's.
Just turn off wireless.
Everyone in the same LAN segment will have no issues finding the TC and backup via ethernet is no problem.
But TC is really not designed for business use.. it is a home device really. It is slow and sealed .. no access to hard disk and no replacement parts.
Synology and QNAP both make excellent NAS that work fine with TM.. yes, they will be behind Apple sudden changes to the AFP file protocol and so stop working but both companies have large installed base and can provide firmware updates fairly rapidly. They are also designed with service and maintenance in mind. ie you can replace power supply and hard disk if necessary instead of throwing the whole box away.
And for the same capacity they are equally priced.. but better because they can have raid provision using two disks.