As long as its only around freezing it will be fine , it generates enough heat to not be an issue for foundations,
As long as the trench is dry and frost free at the pour you should need to worry unless there is a lot of rain or snow on the way before it sets up
if your drilling it as it needs longer to set the mix can be adjusted but the thicker the base the longer you leave it before drilling
Incorrect, thickness makes little difference to setting and curing at these sorts of thickness,if your drilling it as it needs longer to set the mix can be adjusted but the thicker the base the longer you leave it before drilling