I think Rigeld has the right of it. In an earlier conversation about Infiltrate, it seemed clear to me that deploying Infiltrators happens wholly after the deployment of the bulk of your army. (Meaning point 1 is moot, since you do not have the option of deploying a unit with Infiltrate during your main deployment portion, as Infiltrate no longer appears to be optional.) The book says "units with [Infiltrate] are deployed last, after all other units (friend and foe) are deployed." (p38) "When deploying their armies, players can choose not to deploy up to half of their units...keeping them as Reserves..." "During deployment, when declaring which units are kept as Reserves, the player must clearly explain the organization of his Reserves to the opponent." (p124)
I'd say you don't make the choice of whether or not to Reserve them until you have the option to MAKE that choice, which does not occur until "all other units (friend and foe) are deployed," barring other Infiltrators, of course.
Still, good question. Presents some interesting tactical flexibility.