Hm...Timo--on your theory, I always thought that was pigment, not a blush response, more like how animals of some types have one gender that has markings that are intended to indicate sexual desirability to potential mates. And the women, I'm thinking, probably still accentuate this area with makeup.
Oh, and with lipstick--I think I've seen Cardassian women use colors other than red. I've seen some (Makbar comes to mind as one example) go for a more Goth color scheme, and apparently this does not have the social suggestion of "weirdness" that it does on our world.
Myasishchev--A good point about Kira and Tekeny! I didn't even think of that, but that's a WONDERFUL point of evidence about Cardassian blood being red.
With the blush response...my personal theory is that they do--that they would FEEL themselves blush, but no one else would ever see it due to the opacity and/or thickness of their skin. (In my own works, if they're standing close to someone, they might feel a change in the other person's bioelectric field, that is their equivalent of seeing someone blush.)
About the Cardassian metabolism...as therapsids, I'm not entirely sure to what degree their bodies are able to produce heat on their own, and to what degree they must rely on the environment. One interesting fact I picked up in "Emissary" was that the temperature in Ops apparently got stuck on 32 C (89.6 F). That may well be a Cardassian's optimum temperature. If that's the case, then what humans consider optimum would be sheer hell to endure!
And believe me, I know personally just how miserable Garak must be, because my own optimum temperature is quite high compared to most people. I am perfectly comfortable in temperatures ranging from 28-30 C (82-86 F), and I don't like being in places with "normal" air conditioning during the summer...that's why I hate wearing short sleeves, because as soon as I go indoors, I'll be cold. (Conversely, after being in a place like that, coming out into the summer heat--and better yet, the hot car--feels GREAT.) And believe me, if I were on DS9, I would almost surely be showing up at Garak's shop asking if there was some kind of lining I could get on my uniform so I wouldn't be so damn cold all the time!
(And undoubtedly the response would be one with quite the sparkle in the eye, followed by, "I know JUST the thing--but are you sure you don't have some Cardassian blood in you?"