Pls check Dienekes´s thread on the same issue ( http://dienekes.blogspot.com/2010/10/y-chromosome-and-mtdna-of-louis-xvi-of.html ). I´ve found and show there convincing evidence of 3 more back steps in his maternal ancestry tree to a Claire de Brisehoule.
From Louise de Mareuil to Claire most matrilineal ancestry is from Normandy.

2.”Because of social (primogeniture) and economic (Malthusian era growth rates) pressures many of these offsprings of elites would descend down the ladder of status”.

I do not think that was the case. Hochadel, High Nobility in general was almost a caste. Until recent times (XX century) every offspring, even bastards, were kept under control: first sons got all the property and married within the “caste”, second sons and others if lucky could marry a rich bride and stay within the “caste”, if not they were deviated to war (where they died young or if succesfull earned enough money or prestige so as to stay within the “caste”), or high church positions. You can study any of the High Nobility lineages and you will find the same pattern everywhere.

On the other hand i agree totaly with your suggestion:

“A better map of the genes of the European nobility, their patterns of relationship, as well as extractions from tombs and mausoleums, would give us a very fine-grained understanding of the demographic parameters of this population. Though I’m not sure that some of the European nobility would be totally open to finding out the facts, as opposed to the myths”.

Assuming it is Louis’s blood, the maternal haplogroup shouldn’t be that much of a surprise- there are known cases of Central Eurasian admixture into the noble houses of Europe. Probably the most notable would be through Elizabeth the Cuman, a Turkic chief’s daughter who married Stephen V of Hungary. Her daughters married into the ruling elite of Serbia, Byzantium, and Naples, and her maternal granddaughters married into the French, Spanish, and Italian elite.