We hesitate to guarantee anything because rental systems can sometimes behave in unpredictable ways, especially around carve-outs like the 4-hour grace period for the Amex Plat, but our guess would be that if you originally picked up the rental at 5pm, and were booked with the Amex Plat CDP, you'd have until 9pm to return on your extended return day.

As for your other questions, it gets more into speculation, but we'd guess that additional days would be charged at the $43.75 per day rate. Typically Hertz would not call the police for a rental that is 2 days late, but given all of the high profile cases that have been in the news over the past year or so with customers being arrested, that is definitely NOT something we'd recommend taking a chance with.