IMO, Simon as narrator should sum the 14 (standard resistance for magic) + the added resistance for the feat used + 3 (the number of spells, pardon, of feats used to view the scenes which are cast simultaneously) + the assassin illusion ability (as an augment! not the full ability) and match the result against your IO combined might.
According to the result of the contest, your caracter can discover the illusion (major or complete victory) or not (minor or marginal victory) or doesn't see the event at all (any defeat).
Gian
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