Try the American Battle Monuments Commission website at < http://www.abmc.govhttp://www.abmc.gov >. Click on "About ABMC Cemeteries" then click on "Ardennes". You'll also find an address and phone number to obtain more info but don't expect a lot. ABMC deals only with Americans interred in ABMC Cemeteries overseas.
Many of the records you seek don't exist (and most of those never have). You'll find no death certificate filed anywhere. A standard form, Report of Death (WD AGO Form 52-1), was executed (an original and ten copies) and filed with all concerned offices and agencies. One copy went to the designated next of kin and was used in matters which normally require a death certificate. Due to wartime security and other factors, the form contains only the barest of information. For example, under "Place of death", it typically reads "Mediterranean Theater of Operations", "Southwest Pacific Area", etc.