In the wake of media and state probes into executive pay, Westchester Medical Center is overhauling policies after government regulators questioned millions of dollars in bonuses it improperly paid to senior managers from 2013-15. The Valhalla-based health system, of which MidHudson Regional Hospital in Poughkeepsie and the Kingston-based HealthAlliance of the Hudson Valley are a part, paid out $4.6 million in bonuses to 18 employees without keeping sufficient records to show why many received the payments, according to a report by New York State Comptroller Thomas DiNapoli's Office.