Can a 172 flying at 200 feet above water across Lake Superior get through
undetected by radar or AWACS? Are there holes in radar coverage?

You would have a good chance of sneaking through radar but if an AWAC
were monitoring the area then you don't have a chance. When I lived in
the middle east they could detect pickup trucks smuggling Scotch across
the border from Yemen.
--

Tony Roberts
PP-ASEL
VFR OTT
Night
Almost Instrument
Cessna 172H C-GICE