Very generally speaking, 26cm waist width is too wide for size 10.5 feet. Bigger boots will not make much (if any) difference at all - you effectively pressure/flex the board with your feet, not the boots.
It is a common misperception that too little overhang is preferable to too much. The opposite is the case for for pretty much everybody but the most hardcore Eurocarver.
i think that's subjective honestly. the snowboard site i got it from seem to know their thing.
but anyway, judging by this photo, does it really look like my board's too wide?
Oh, and @bseracka, I'll try to fix that problem and take a new photo.