Hmmm.... Well, A full defense takes an action. Actually, by RAW, you should not be able to take two Full Defense actions unless you have more than one Initiative pass. That being said, I would have to go with ruling number two. If Full Defense uses an action if it is used as an interrupt, then you have to have two actions to do so, and you give up two actions.
Full Defense is not a continuation at all.
As for quotes from RAW:
"A character may invoke full defense against an attack at any point in
a Combat Turn, so long as the character is not surprised (see p. 165).
This means a character does not necessarily need to declare a full defense
and take a Complex Action in advance—he can instead declare
a full defense when attacked, even if it is not yet his Action Phase in
the turn. Going on full defense as an interrupt, however, uses up the
character’s next available action."
Bold font mine. Note that teh RAW also state that Full Defense is a complex action. This means that it cost you your whole action if you declare this on your turn. Also note that full defense is just a declaration. How you use that full defense is dependent on the next attack against you. That means that if you declare that you are using Full Defense on your action, if you are shot at you will use Reaction + Dodge or Reaction + Gymnastics.
If you are attacked with melee then you can use Reaction + Dodge + Dodge or Melee skill * 2 or Gymnastics + Dodge.
Note that Gymnastics can be used against Ranged or melee but has no multiplier like Dodge or Parry in full defense.