2. For the modulesystem (ModSys, yes) side of things: in module_presentations.py search for
("game_multiplayer_admin_panel"
. That should lead you to the start of the admin panel presentation, and in the ti_on_presentation_load trigger of it there is a decently long try_begin/else_try/try_end block that assigns a reference to a mesh to a variable based on what scene is currently selected in the admin panel. You can extend this block with your own else_try to add scenes by yourself.
I'm afraid I can't really help you with actually getting your map screenshot into a mesh and adding it to the game, never done much myself in that area. I guess you could try looking around the files for a bit and try to figure it out yourself or wait for someone else to help out.
3. I'm not sure how NW detects changed modules, sorry.
As for changing the name in the admin panel, that is quite simple: Open strings.txt and search for the original name (make sure to replace all spaces in the name with underscores (_). You can then replace the name with your chosen one, like this:
Code:
#old
str_mp_ambush_fog Ambush_(Fog)
#new
str_mp_ambush_fog Training_Camp
As for the preview image, you should replace the one used for the map you are replacing (if there even is one). You can find the name of the mesh in the code I described regarding question 2, and then maybe you can figure it out from there - Again, I sadly can't really help you here.
That said, having a preview image for your map really
isn't important - I don't think I've seen many scenes made by anyone but TaleWorlds that comes with a preview images.