BT,
If the entire surface of the lake or the entire width of the area you were crossing were slush I think you'd be advised not to even go there.
I do believe you are correct in being worried about getting stuck. I would be too. If Winni had more that just occasional pockets of slush I would have headed for shore.
If it's pockets of slush I think some pretty good advice was given here. Good Luck out there!!