OPTIONAL EFFECTS

When exactly are optional non Order effects decided, like Swift Strike?

Any effect that has a mandatory trigger, but optional language ("this unit may") must be decided in the Declare step. In the case of Swift Strike, you must decide if your unit will retreat when declaring after attack action abilities and triggers, not while resolving them.

Must abilities that state that they may be used "once per game", be used on the first possible instance of that trigger?

No, their owner selects when they are used. (This is regardless of the presence of language indicating they are optional -CFAQ Ed.)