[AngryAlpaca]

Whew! OK, since you are definitely knowledgeable, how about using styrofoam instead of neoprene? It has a better R factor, and you can carve it close enough and then compress the last bit.

Styrofoam won't get in as close as neoprene would.

I've come across some unsubstantiated information that states that a TEC does NOT spread heat, it carries it quite linearly.

Makes sense to me... Hundreds of tiny thermocouples spread across, with only a 1-2mm spreading area, so it will probably be fairly linear. If it spread well, then what would be the point of the coldplate?

what's the resulting area that actually requires cooling? Is it less than a typical 40 mm by 40 mm TEC?

Why would it be less? Yes, most of the heat would spread to the coolest point of a waterblock (typically the middle) but there's still a bit on the outside, and cooling a large area removes any guesswork. Hell, get an MCW6000, then you aren't sacrificing anything (except appearance, supposedly.)

Here's another mystery: does the cold plate bend or flex, when clamped, and by how much? Does it impact performance?

I'll say no. The coldplate is clamped against the TEC (flat) which is clamped against the waterblock (it damned well should be flat.) Using one flat surface to pull another flat surface straight towards itself will cause both flat surfaces to remain flat. If the waterblock isn't flat, however, then both the TEC and the coldplate will bend a little bit, hurting the thermal transfer.