Paul, Not solved, but I do have a better temporary workaround.
I unfortunately did not have much time to dig into this issue over the weekend. I did receive the replacement Westell modem/router with no change in outcome as expected. I also quickly realized that the Westell 7500-44 modem/router does not support WDS so my debug plan with the borrowed airport express was shot down. I currently have the gateway connected through a Netgear WN3000RP range extender to my wifi network and it seems to be operating as expected (past couple of hours).
My working theory all along is that the gateway's signaling on my particular GW device is not clean (or marginal) and was creating havoc with the Westell, even though the Netgear seems to be able to deal with it. I'm sure the result with the airport express would have been similar. Although I'm sure the Westell is probably not of the highest quality for its own part, I've read here of at least several instances of Westell 7500s operating with no issue with the TED and since I've tried two different Westells here which seem to handle everything else I've ever plugged into them properly, I'm now more suspect of the GW at the moment.
I would really like to swap out the GW to see if it follows the GW as the Netgear intermediary device is not a good long-term solution for me. I suspect my GW is either out of spec or marginal. I will try a few more experiments for due deligence when I get a chance but, barring a major breakthrough, will likely contact TED for a swap-out.
Opinions or thoughts on further simple experiments are welcome, but after I've narrowed it down to what I consider "reasonable" doubt on this particular GW unit, I don't feel nearly as motivated in spending another 10-20 hours trying to get to "beyond a shadow of doubt" status.