The first step in solving mistakes and problems is to recognize them, and that is clear to you.
My two cents: Do not be in a hurry to finish the map. Try to get everything you've proposed. In this site you have many tutorials and help to learn and thus to be able to solve those faults that you comment.
a few tips.
add some debris(not buyable debris but detail debris) so it doesn't just look like open areas and you cant just train for days anywhere
don't use textures with _blend at the end as the main ground texture, thats why you can see the zombies underground.
if that is a custom texture just don't make it a blend texture.
if you move the stg prefab out .25 it won't keep colliding with the wall.
its coming along pretty good keep it up man!
map look good