The Hed SHOULD work, but you'd want to try it to be sure. I had a set of Hed C2 90's (which is the same rim bulge profile on that Hed disc) & it fit on my P3C that I had. I have heard about other people that had clearance issues though. On mine there was ~2mm between the widest part of the rim & the inside of the chainstays.
Carbon chainrings for a TT bike would be sweet!
Those Edge 1.38's sound like a solid build, but I've believe that the aerodynamics of them are pretty bad. The basics of rim aerodynamics are that if the sides of the rim bulge out (like Hed & Zipp) then they are fast rims. Rims with straight sides (Reynolds 46's, Lightweights) are "bad" in the words of some respected aerodynamic experts in the industry (one of which is pictured next to Sastre in the wind tunnel on velonews.com). Rims whose sides actuall curve inward, well, it just gets worse. In my opinion the 1.38's are great climbing wheels, but that's about it. The 1.45's & 1.68's are much better. Just sayin' that if going fast on the flats is the main concern then there's the potential to upgrade.
Your bike shop is letting you borrow a Ventus LTD? Wow. You must be a great customer! Just be sure that you can get it to work with your position on the bike! Some people can't get it to go low enough. I know that some guys & girls on the Cervelo Test Team are having that problem with the Ventus & are having to get custom modifications made.
Have you ever wondered if there was more to life, other than being really, really, ridiculously good looking?