MrGrey

You'll need the zombiespawnmap as you suggested to designate the areas and density the zeds will spawn with white being the highest spawn density and black being no spawns at all. Initial zed population and respawn rate can be altered in the servers sandbox settings. Obviously answered quite a while after your initial post so hopefully you'd found an answer to your question already, but if not, hope this helps!

Hopefully you've sorted this by now, but if not, how are you trying to import you png/bmp files? If its asking for pzw files it sounds like you're trying to go into the the file tab and open them that way which won't work as that's only for opening up saved map projects. To add your png all you need to do is create your world map with however many cells you plan to use, then drag the png directly from the folder its in onto the cell. Make sure you also have a saved '_veg' png/bmp for the map as well or it won't allow you to add the base png/bmp image Hope this helps!

I can't remember the exact name of the setting as I'm not at my computer at the moment but in the 'View' tab there is one that toggles your buildings to only display the ground floor tiles like you have in your picture. Helps when you have to do any alterations behind the buildings that you wouldn't be able to see with them fully visible. Toggling that should bring your buildings into view