Under the Dual Sovereignty Doctrine, even if he is fully acquitted in Federal court, he can later be charged and convicted in State court for the same actions; this is not considered Double Jeopardy.
Double Jeopardy applies to the offense, not the act. It is an offense against the state to violate its law against homicide; it is an offense against the federal government to violate its law against homicide. The same act creates two offenses; each can be prosecuted independent of the other.
(I haven't agreed with this doctrine in the past, but this particular case has forced me to consider factors of which I was previously unaware.)
No. The "reasonable person" standard is applied. Would a reasonable person in the position of the imperiled person believe they faced a credible, criminal, imminent, threat of death or grievous bodily harm? If so, anyone present may use any level of force that a reasonable person would believe necessary to stop that threat.
Even if we give him the benefit of an unreasonable doubt and say he was sufficientlt imperiled, "Necessary" is what is going to hang this guy: the level of "force" "necessary" to end the "imminent threat" was to take a half-step to the right. "Sidestep-Right" is the extent of the force he was justified in using against her here.