This is how WE play it:
1) The spell is cast (with the SCSM, if necessary) when the spell is stored
2) No roll necessary when it is "released"
3) The spell is cast as an instant spell
4) The staff spell DOES count as a spell vs the "max one spell pr round", so no other spells may be cast
We did a slight modification for attack spells, since we don't want the caster to know in advance how effective his attack is going to be. So for attack spells, the spell was cast as mentioned above, but only to check if the spell failed. If if DIDN'T fail, the spell was assumed to be successfully cast, but an attack roll was made when the spell was "released". For that roll, 01 and 02 results were rerolled, since the spell did not initially fail.
Also, if a situation arised that would make it harder for the caster to cast the spell (like strapping on armor after storing the spell), we decided that a new SCSM had to be made even when releasing the spell.