Is there a formal order to declaring what you're using and what you're using for bonuses?
With magic use you don't want to go over your threshold, but you do want to maximize your chances to cast. It's a fine juggling act and often you have the option to channel or add Augments (Fey Sight). It seems sneaky for an opponent to decide to go with whatever would subtract less just to get you to go over your threshold. For that matter, you couldn't you just decide not to use shields or other bonuses you get as well?
Ah, we had be resolving the spell as part of the combat action rather than as a separate roll.
For the combat situation both players just do it simultaneously. Its rather quick and we have found that players are rather quick at it even if you decide to you need to do some coordinated bonuses to counter your opponent.
As for magic. It uses the following steps. (im on page 28 Core Rules)
Roll a single die for your power.
Cast the to see if you successfully cast it. This is where you do your channeling etc to make certain you beat your difficulty. If successful move to step 2.
Determine the combat situation and use the same die result you rolled earlier to see if you hit. Apply the Combat Situation to the die roll. (channeling etc is only used in step 1)
Resolve the power's attack.
The big point here is that the Combat Situation enhancements do not apply to whether or not you successfully cast a power and channeling does not affect step 2.
Step 1 and 2 use the same die roll.
In this way what the sneaky opponent does will not throw you over or under your ability to cast the power, only whether or not they will be affected by it.