Welcome JK.
Don’t know about the ice on BC, I havent driven past in a while. As for when the ice will come out… keep your eye on the weather.
There is more then one sunken roadbed in BC. Get ahold of the Hotspots Saylorville/Big Creek map and you can see where they are. To be best of my recollection, they are not at the south end where the water is deep. From memory, I think one cuts across the wide part and at least one, or two, cut across the the northern arms. I think there is also the remnants of a sunken bridge. Here is a topo, but you really need a better map to see where the roads are:
http://www.iowadnr.com/fish/fishing/lakes/maps/bic77.jpg
Good luck,
Dave