Q1:
This feat has eratta specifically disallowing the Improved Trip free attack.
Q2:
I think Eyes in the Back of your Head wins, for two reasons.
First, as a general rule, defense trumps offense in 3E.
Second, the mention of the "+2 bonus" seems to me to be for clarification purposes (to distinguish it from the ability to sneak attack) rather than qualitative ones.
That is just my opinion, however.
Q3:
I thought Fists of Iron required a standard action to use, but I have no rule support for that theory.
Q4:
DM discretion for all questions. The rules cannot cover all permutations of these kinds of situations.
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gnfnrf