Yes the harbours were prepared for demolition but the orders were for this to be damage that could be fixed in a week, as it was expected that they would be back in British hands pretty quickly. There is evidence to suggest that the Brandenburg commandos were supposed to attack Dover port frontally from fast boats to prevent it being blocked but that was surely a suicide mission (and pointless if the port was not under attack from the rear as they wouldn't be able to hold the port by themselves). The cranes might be destroyed but many of the German ships had cranes on them. Also, the paratroopers weren't going to drop on any ports, they were going to land near Lympne airfield and attack the ports from the rear in conjunction with the troops from the beaches. A 70% casualty rate was normal for most WW 2 paratroop drops, even the successful ones.