The biggest problem ausf, is that the particular Sherman you are building is a very late production version of the 105. From looking through some of the references I have, it did appear in the ETO late during W.W.II. I have some pics of them entering Germany but really haven't found anything earlier yet.
So in answer to your three questions:
1. Yes the U.S. had some camouflaged vehicles in the ETO. Usually black over O.D. but on occasion some where 3 colours such as black, light green, and O.D.
(Haven't seen an M4 105 HVSS done this way though)
2. As for camo nets, just about anything goes. Drape away !!!
3. As for the Ardennes, like I stated earlier these were very late variants an may not have been at the front yet (but not 100% sure). Most captured vehicles were gun tanks. Because the 105 was not a front line vehicle the odds are greatly reduced the Germans had many of these. Most captured Shermans I have seen are of the 75mm, 76mm and Firefly variety.
Question is, is your heart fixed on building a W.W.II variant? If not then the options are greatly increased. Even have a couple of pics of them captured in Iraq.
Happy Modelling and God Bless
Robert