Yes, ParaSait is correct. There are some issues with the LOS shooting code which ended up with the corner shooting "trick" that is embedded in the game play that fixing it is non-trivial.
But back to the suggestion. =P
I think that half-seen squares idea sounds useful but might be unlikely to make it into production code. The reason is that you can always shoot at any of the dark squares "safely" -- the moment you fire into some place that you cannot see completely, you will automatically incur the 50% accuracy penalty. The only reason why you might be unable to hit something particularly when you fire from the south is due to the strange LoS code that we are currently using.
As for showing the spread for shotguns, I like that idea. (=