We've always played that the spell was completely cast, but doesn't have answers to the Target and Range portions of the spell (basically). Pretty much what was said earlier. No roll to release the store.
If our Mage wants to SM a Teleport to affect +1 target, then store it in his Magic Staff, no problem. As it is released, as an Instant for us, he choses the additional target in that 10%Act just like he determines destination. He may use Spell Concentration, Magical Language, Spell Prep, whatever else he can squeeze into his 1 minute of Spell Store time, but not once it stored.
Now, if you're using anything that will "Parry" a spell (different Lists), or Cancel spells, modify RR's vs opponents/attackers, then all that info is set in stone and unalterable while it was being placed into the Spell Store device.
Also, Spell Mastery after casting would open a huge can of worms.
Would anyone allow a Daily X to be Mastered after the fact? I wouldn't.
I would allow Spell Enhancement to modify "the next spell cast", which could be from a store.
For us, it mattered when the PP "ping" happened for Power Perception/Awareness.
IMHO, armor penalties should be factored in when the spell is cast (right after the Spell Store) and not when it is released from the Store.