A couple of questions on the turning up of an Event card and its execution...
(1) When an event mentions the "nearest" unit, is the unit being attacked considered vulnerable or is it the nearest to THAT unit that's being targeted?
Not sure how you are using "vulnerable" here, but determining the target of the event begins with the unit being targeted (and may include the unit being targeted) then goes out from there.
(2) Do the secondary events (actions) occur on a non end-of-turn flip if the primary event doesn't apply? (For example, a valor check results in a card flip: breeze, but not applicable; so do you instead do the secondary actions?)
Yes, this is correct. In fact those secondary events originally came into the design because players disliked how infrequent the events actually fired. So, while it was inserted to address that issue, it was a happy byproduct that it also created counterattacks and overload turns as well.