None

Roboto

-

Eolk

If your skins / maps come in a .vl2, then just drop them in your base folder, and you're done.

To make a map, you might want to grab a blank map. Start a listen server with the map and press ALT + E.

If alt + e doesn't work for you, type...

exec("scripts/editor.cs")
toggleEditor(1)

in the console (~tilde) with semi-colons at the end of each statement.

[EDIT]
Additionally, here are some other good resources:
http://totalk.ca/lexor.new/downloads/T2_SSSK_rev3.zip
http://tribes.necrobones.com/tribes2/tutorial.html

Fling

tribes uses .vl2 files which are basically zip files with the file structure intact. You just drop a .vl2 file in base and the game will automatically extract it to where the file structure tells it. Sometimes that doesn't work though, and then you just extract the files to the proper folders. In the case of skins, gamedata/base/textures.

Eolk

Mapping usually requires some scripting knowledge if you want to change the skies and terrain, because you have to edit the mission file directly.

Yes, you can play construction solo and LAN. Launch the game in LAN mode and host a listen construction server.