will not start (new battery) locked in gear (park).

What is your question?

Applied jumper cables and alarm went off and all electrics cycled on and off. Still, did not start or turnover. Could it be the brake sensor switch or its fuse?

What makes this problem better or worse?

It's worse when I'm cold. Brr!

How long has your 2015 Ford Flex had this problem?

a few weeks.