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Re: On the subject of mysterious absences...The Answer

"Opinion"? Singular? :-)

The origin of the turtles is still unclear enough to be worthy of a Ph.D. thesis. Mine, to be precise. (And no, I don't have any results yet -- I'm still building the framework of the data matrix.)

I know I can't do this question justice, but many recent phylogenies have shied away from the testudinate-pareiasaur link,

Though all of these build on the same analysis, more or less, and most or all of them have insufficient taxon sampling...

However, AFAIK no study has nested turtles inside Archosauria (or even Archosauromorpha),

No morphological one. Molecular studies have done both. (Obviously, however, molecular analyses inevitably suffer from rather ridiculous taxon sampling.)

though certain analyses have found turtles to be lepidosauromorphs.

Either as sister-group of Euryapsida or as sister-group of Lepidosauria. So these are two hypotheses.

For example, one recent paper recovers a clade that includes turtles and sauropterygians (placodonts, plesiosaurs, etc) as the sister taxon to a Kuehneosauridae+Squamata clade (Holmes et al., 2008; JVP 28: 76-84).

Though the focus of that analysis was elsewhere, and the turtles were a single terminal taxon...

As for "Thecodontia"... it's a useless term. :-) For the most part, "Thecodontia" was a grab-bag of archosauriform taxa that could not be conveniently assigned to any of the "major" archosauriform groups (Saurischia, Ornithischia, Crocodylia, Pterosauria).