About the placing over enemies: Wall spells that don't allow to place the spell in an occupied square will usually tell you so, like Wall of Stone. As a rule of thumb, spells that create a wall that characters can walk through usually allow you to do that (there are exceptions, like the Prismatic Wall). In this respect, since there is a precedent, not saying you can't means you can.
The Wall must, by exact wording, be a straight. Since its area can be as big as 10' per caster level, it can be any type of rectangle that stays within those bounds; "up to one 10-ft. square/level" means that that's your max size, it doesn't necessarily have to be the full size.
The placement depends on how religiously do you take the map grid... If you live and breath by it, then you must always have it occupy an area that covers all the grid squares it's on completely, so that there are no partially occupied tiles; this means no diagonals. If you allow some leeway, you can place it anyway you want, and characters in a tile partially covered by the wall would either have penalties to their actions but be unaffected or be affected but with a lowered save, at the DM's discretion.