It pretty much does that for interior maps, but outdoors it's pretty much just limited by distance. Will have a think about solution, possibly trying to draw the arc might be easier. Throwing over 1 story buildings is likely to be feasible, but taller buildings less so.
Concerning skill, they can miss by up to 3 tiles, depending on how severe the miss. Difficult to say why they prioritise Sam, they shouldn't if there's more potential for damage elsewhere, also since there's no indications if they missed or not, it's hard to say what their intended target it. Anyway, have reduced explosives skills a bit, and reduced the number of grenades specialists have (except for fanatic, made man and NCO).