Uhm... the phases of a Combat Round are:
1) Declaration
2) Initiative Determination
3) Actions Resolution
To be able to declare in reverse order of initiative you have to switch phase 1 and phase 2 (in addition, the declaration of some actions modify your initiative, so... it can lead to strange results).
I've always played with "simultaneous" declarations (first the GM secretly selects the NPC actions, than the PC declare their actions).
Anyway, if you have declared the casting of a spell, but you are stunned before your action, you can still change your action to "Full Parry" (with half OB), right?