Last Created Unit is a global variable which automatically stores the value of the last unit created via "Unit - Create Units..." action.
As for the summoned unit, didn't try but it cannot pick all the illusions anyway. However, everytime an unit casts the ability, wait until it finishes casting it (dunno the time, experiment with it) and then just pick every unit within the area of the caster which is illusion andwhose unit type is equal with the one of the caster. Normally that should do it.
~Daelin