The reveal plots step is broken down into 3 sub steps.
1.3.1 Compare initiative.
1.3.2 Choose first player. The reaction on Cersei's Wheelhouse is triggered and resolved here, before the next step.
1.3.3 Resolve "when revealed" plot abilities.
Also, Cersei's Wheelhouse cannot result in a negative initiative value.
A quantity cannot be reduced so that it functions with a value below zero: a card cannot have “negative” icons, STR, traits, cost, or keywords. Negative modifiers in excess of a value’s current quantity can be applied, but, after all active modifiers have been applied, any resultant value below zero is treated as zero.