I believe you have the syntax correct-
getareausers("<map name>",<x1>,<y1>,<x2>,<y2>)
This function will return the count of connected characters which are located
within the specified area - an x1/y1-x2/y2 square on the specified map.
This is useful for maps that are split into many buildings, such as all the
"*_in" maps, due to all the shops and houses.
I've only ever used a coordinate plug like that for setcell but it looks very much the same.
setcell instance_mapname("1@payonn"),216,253,216,247,cell_walkable,0;
and I went left to right (x) top to bottom (y) and I believe that worked fine. I'm not actually sure if you'd have a problem doing the reverse. I'm also not sure I understood the last question- if you are asking if the coordinates themselves are included in the area check, again the answer is yes.
Hope this helps,
~Azura Skyy