Very sad. I wonder why they turned north.
Yikes! My family has a summer cottage right in Ludington, in the Epworth Heights community. Lovely area. I went every summer until about 1988 (13). I’m sorry to hear about a plane going down in a piece of water so familiar to me.
I doubt that they did. Everything after 7:13 in the track log seems to be an anomalous reading unfortunately picked up on by the press.
The real question is why they didn’t put down at Ludington. Born and raised in Ludington I can tell you the airport is not that hard to locate!
I’ll have to share my tale with you sometime of how I flew in there on a daytrip about a sunken dredge and wound up snowed in for a week!
Getting chills looking at the altitude/speed chart. They went from 29 kts to 127. 
More information on the people on board
http://www.nytimes.com/2010/07/24/us/24brfs-PLANECRASHES_BRF.html
The FAA reported groundspeeds are sometimes erroneous in turns, I wouldn’t read too much into that 29 kts report.