William is holding his wife's cloak. In the very next shot, she's wearing it.
When Seth is pulling Sibella up from the water, there's not enough tension on the rope to have a person on the other end.
About 38 minutes in, a grumpy brunette woman tells a man---whom she is accusing---"you and she". The correct wording is "you and her", as she was not using the nominative case.
The villainess arranges accidents designed to make it look as though someone is trying to kill her. The fall that she arranged through the boathouse floor would have killed her.