Partial disagree on the edit:
"All assignments of characters are on a one-to-one 
basis, with the following two exceptions:
• If your assigned companion has the keyword 
defender +1, you may assign that character at 
this time to one additional unassigned minion. 
Defender +2 allows that companion to defend 
against two additional unassigned minions, and 
so on. A character with defender +2 (or greater) 
satisfies any requirement for defender +1.
• When you have informed the Shadow players 
that you are done making assignments, they 
may assign any leftover unassigned minions 
to any companions (even if those companions 
are already assigned). The first Shadow player 
on your right may assign any of his unassigned 
minions, and so on, counter-clockwise around 
the table."
The way the AR are describing it, the companion is only being assigned to a skirmish when the FP player does the assignment, so it would trigger for a defender+1 second assignment by FP.  However, if the shadow player is assigning an additional minion to an already-assigned companion, the companion is not being assigned to a new skirmish: the new minion is being assigned to a skirmish.