Hmm. Those links in the other post to "Fraudulent QDROs" are interesting. Hopefully my 401(k) plan admin will see that they don't apply to our case, as I’m not trying to obtain a final distribution, just an allowed in-service withdrawal. Anyway, as it turns out, a resolution may be simpler (or more complicated?) than I originally thought...
On review of my wife's copies of our divorce paperwork, I found a letter from her attorney that included a copy of my employer's response to her joinder of my
company pension plan. In this letter the attorney recommended that my wife pursue preparation and service of a QDRO. The attorney's involvement apparently ended at that point, and my wife now confirms that she never followed up on her attorney's recommendation. In other words, there is
no QDRO against any plan I have with my employer. (Sorry for the confusion, but I was only taking my plan rep's comments at face value.)
At this point it appears that my 401(k) plan admin is
guessing about the existence of a QDRO applicable to this 401(k), based on the response that my company
expected from my wife to their response to her joinder to my pension plan.

Complicating this is the fact that my company was sold to another company after our divorce and remarriage, and the original joinder paperwork may not be readily available to the current administrators of these plans...
Anyway, IMO, the apparent assumption of encumbrances on my 401(k) based on a QDRO (assumed or otherwise) against my pension plan is unwarranted. Any 401(k) assets should have been dealt with in a separate joinder or explictily included in the original. In fact, I wasn't even participating in the 401(k) at the time of this settlement, so my this plan admin’s apparent concerns and opinions in this regard seem to be without merit.
The terms of the joinder to my pension plan requested that no payment or distribution be made without my wife’s prior written consent. Although (IMO, for reasons stated above) that joinder does not apply to my 401(k) plan, I see no reason why this plan admin should not immediately respond to a written notice from my wife explaining the facts of our divorce and remarriage, the lack of an applicable QDRO for any plan, and my non-participation in the 401(k) at the time of our divorce settlement. However, my concern with such a tactic is that it might set a precedent (in the plan admin's mind) requiring my wife’s intervention for similar future actions in my 401(k).
???