engineered wood floor on radiant heating

hikergirl99

Here are a few questions for those on this site that have lots of knowledge:

1) What substance do you use to make a concrete floor level? i.e. fill in dips
2) A particular 3/8" Bruce floor suggests a particular underlayment that is only 0.075 inches thick. Can I use something thicker? Can it be a different brand? Or is it best to stick with the same company? Will 0.075 inches give enough cushion for the floor?
3) For a transition piece at a doorway that butts up to tile, what is the best piece to use? Can the floor be butted up directly to the tile (with metal edging already in place)? or do you really need to have a space for expansion in the 3 foot doorway?

Thanks ahead of time!