Solution:

Two people.

The question mentions that a couple went to see a wildlife sanctuary. There is no mention that the rest of the people went along.

Solution:

It is simple. The sheriff never told him where the crime scene was. But still he reached at the right place. This points towards the fact that he knew it already and that he is the killer himself.

Solution:

The person is leaving hospital after visiting his wife. His wife is on a life support system. The elevator has stopped due to the power cut. If there was an emergency backup generator functioning well, the elevator would have also be running. It stopped in between means that the life system support has stopped as well.

Solution:

angelina is married to brad.
catherine is married to douglas.
mendes is married to kate.