For the first question, the U.S. naval leader can use his air re-roll for any battle planes launched off of his carrier are in - either the Japanese attack against his fleet or the battle his planes are in in the adjacent space. If he chooses not to use his leader in the Japanese attack against his fleet, he can use his re-rolls in the battle his planes are in attacking the Japanese - per rule 1.5.6
For the second question, you must assign your planes first and then AA is rolled per rule 13.1. Rule 7.0.11 refers to snap fire - but I will change it to reflect the process in 13.1. Air units are always assigned before AA is rolled.