Just like extending the term of the Presidency through amendment would not apply to a sitting president, such an amendment that you speak of would not affect 0bama.
Nope. The amendment would have to state whether it applied immediately or not, and that would take priority over any other law or Constitutional provision - even the one applying to ex post facto laws. The only limitations on amendments were that a state could not have its two seats in the Senate reduced without its consent and that slave trade and direct taxation couldn't be changed before 1808.