Some answers for you:
"Are there any other paint variations other than O.D. green for this variant? i.e. such as the tan/O.D. scheme in Italy or black O.D.?"
The M4A3 was a later entrant to the European battlefield -- most likely after Aug '44 timeframe based on era pictures. I don't recall units in Italy getting them. The Black/OD scheme was in used up until the end of the war in NWE.
"Was this variant involed at all in the breakout from Normandy so I could put a hedgerow device on the front?"
Nope. The M4 (105) is seen but this is fairly different than the stock Tamiya kit (which I love, BTW). As I remarked before, the M4A3 (105) comes in theatre after most of the Battle of France has ended. No Cobra/hedgerow fighting for the M4A3 (105).
"Are there any AM decals or dry transfers out there to give me some different options for markings?"
I think Archer has some 6AD dry transfers.
"The large "chevron" treads on the tracks were made of rubber, correct?"
The T48 chevron tracks were all rubber, but there were several steel chevron tracks too. For a good reference, go here:
Laughlin ArticleHope these help.