I think my foot is as bad as or worse than yours, flat and overpronated
Sidi Mega did not fit me well, the shoe may have been wider but the sole seemed to be the same as the nonmega
so my foot would rock side to side.

My last 4 pairs of road shoes have been Shimano all size 46, currently use r170
bought an r171 couple months ago but not used yet and have r087 on trainer
the r087 was wide size but others were not and fit me still.

46 is probably just slightly half size larger but works for me, if my foot was normal width I would
surely fit in half size smaller easily.

Normal shoes are a problem for me, I only find shoes in 11.5 that fit me and some dont at all due to lack of width in arch area.