Yarding all of this up, here's my take.
1 - Duration: there needs to be a finite duration on period of activity covered, or the magent can just cast once and be covered forever. The description says it covers deeds performed "while this spell is active" but doesn't specify what that period is. The period of activity coverage needs to be finite, but the effect of that coverage carries forward into the future, and any divination spells cast down the road will see the shadowed figure for any divination within that period of activity.
This gets to be a bit like asking what is the period of activity on a slaying spell. The spell's activity was just the slaying, but the after-effects are permanent.
2 - Activities covered: "All deeds performed" seems pretty comprehensive. Not just the act of putting the dagger into someone's heart, or pouring the poison into their wine. If you do it during the period the spell is active, it's shadowed.
3 - Power against counter-spells: The primary spells (using RMSS, though I am porting the spell into my RM2 game) for this would be Death's Memory (explicitly cited in the Shadow Assassin spell), the Past Vision Open Mentalism spells, and possibly the Vision Behind spells for Seers. Death's Memory shows an "vision of how someone died and an image of the killer," which the Shadow Assassin spell would force to only show the shadowy figure. It also has to be cast within 24 hours of death.
Past Vision and Vision Behind both show events associated with a place or item at a previous time, for a period of minutes to hours. Depending on what item was available or what the place is, this could allow the caster to see the assassin, or possibly not. If the assassin was carrying a dagger for 24 hours before casting Shadow Assassin, then killed someone and left the dagger, a Past Vision or Vision Behind spell going back to that period before Shadow Assassin was cast would see the assassin during that period. If they did not have the dagger, they couldn't trace back, and if they didn't go back to a time before Shadow Assassin, they would see the shadow.
I don't see having a 10 min/level time limit as nerfing Shadow Assassin at all. I don't see allowing it to cover other actions as making it too powerful. I do see making its period of effective concealment unlimited being very problematic. All a 4th level Magent needs to do is cast it and say "this is to conceal my actions leading up to the assassination of the High Priest of Mixor" and then never really try to assassinate that High Priest. They would be concealed from divination/scrying their entire career.