Re: Today Cordillera, tomorrow the world.

Apparently the last
1-2 million years of the terrestrial Cretaceous record is undocumented in the
east,

:-(

and faunal exchange was well underway with the shrinking of the
interior seaway.

Sure.

Ceratopsids were always unknown in the east,

Well, as you write, the eastern NA fossil record is rather poorly known anyway.

but tyrannosaurs entered
eastern America in the Campanian, proving their ability to traverse marine
barriers (presumably via island chains) even when the seaway was at its
greatest extent. How far might they have gotten in the latest Maastrichtian,
when seaways were vanishing?

Not into Africa and Madagascar. Theoretically everywhere else, but evidence (and time before the K-T extinction) is lacking.

It is interesting that "Betasuchus" is similar to Dryptosaurus,

You must have pretty good evidence to suggest that that scrap called Betasuchus is similar to anything particular. I've read it might have been an abelisaur.

which argues for a continued connection of America and Europe via Greenland.

This is clear anyway.

Also, the primitive nature of Thescelosaurus (unlike most Cordilleran
contemporaries) and its lack of clear American ancestry, suggests
immigration, hence contact with a formerly isolated region.

Its lack of clear American ancestry can also indicate that relevant fossils have not yet been found or that paleontologists have so far failed to recognize its close relatives as such (the former possibility is quite probable). And "primitive nature" says rather nothing. Plesiomorphies need not be explained, unlike apomorphies.

Perhaps that was a European island,

Possible, however.

given the tentative identification of Thescelosaurus from Romania.

News to me... I only know Rhabdodon from there.

In addition to a probable America-Europe link, Tarascosaurus, in
particular, supports a Europe-Africa connection, inasmuch as abelisaurs are
otherwise known only from Gondwana.

Another (unnamed, large) abelisaur has been found in France; Erectopus from the EK of France is/was regarded by some as an abelisaur, too, and there was Genusaurus.

All this supports occasional rare immigration from Africa to Europe -- nobody needs a land bridge for this.

Chatter[j]ee noted strong similarities
between the dinosaur faunas of India and Europe.

In particular?

Krause et al determined
that virtually all extant Malagasy vertebrates arrived post-Cretaceous,
undermining the notion of an isolated Madagascar by the latest Cretaceous.

Not in the least. You imply that there are no vertebrates on oceanic islands like Galápagos. Madagascar has been totally surrounded by sea since it ripped off of India. ("Ice Age land bridges" are impossible, the sea is far too deep there.)

Czerkas once mentioned probable South American tyrannosaurs, based on
"fragmentary fossils." This seems unverified,

Indeed, I haven't seen this anywhere else than in "Dinosaurs: A Global View".

but together with Notoceratops is intriguing.

The type and only specimen of Notoceratops is lost, so we still don't know whether it was a hadrosaur or a ceratopsian.

Cousin et al

Who is this? Ref? :-|

wrote "...theropods were briefly the dominant group
before they died out together with the sauropods...c. mid-Maastrichtian."

Nonsense, as others have already pointed out.

That is intriguing as it suggests an alternative to climatic change as the
cause of the extinction of Ampelosaurus, Rhabdodon, and Struthiosaurus.

1. Who still believes in a simple climatic change as the cause for the K-T mass extinction???

2. Do you try to explain the extinction of each genus separately? Good luck :-> !!! What about Hypselosaurus, Pyroraptor, Variraptor, Euronychodon, Pararhabdodon, Rhabdodon, ... which ones have I missed?

Composed mostly of weak defenders,

Defenders. The Great Battle about Euramerica. Sorry, evolution and extinction don't work like that.

the w. European menagerie seems highly
vulnerable, and its demise foreshadowed extinction elsewhere.

As I wrote you (have you read it?), in the last few million years before the K-T, when the sea level sank, a new type of vegetation appeared in southwestern Europe, and hadrosaurs like Pararhabdodon seem to have replaced the titanosaurs. AND THIS FAUNA REMAINED THERE HAPPILY EVER AFTER till the meteorite struck a few million years later. How does this foreshadow extinction?