Nobody seems to have suggested what I would have thought was the obvious solution. Take the bus from Plakias to Rethymnon the previous afternoon and spend the last night of the holiday there. It's an interesting town anyway, has a wide choice of eating places and accommodation costs from 10 Euro a night (hostel dormitory) upwards. That way you can catch a much earlier bus to the airport.
Rethymno - Heraklion:
06.30, 07.30*, 07.45, 08.45, 09.45, 10.45, 11.45, 12.45, 13.45, 14.00*, 14.45, 15.45, 16.45, 17.45, 18.45, 19.45, 21.15, 22.15
*Via the Old National Road
Missing the flight could prove very expensive indeed and surely can't be worth the risk.
Allan