This line is here to let you know how to fill your csv. The "TOKEN" column is used to find the string in game, so you can fill it with any identifier you want (for example "M_LOADING" will override the "Loading..." string in loading screen), this is the one you will set in ndf files. The string which will be display is the one under "REFTEXT" column. There is a more explicit explanation in the documentation.
You can edit "EmblemTexture", "PortraitTexture" and "TypeTexture" (this one is used to describe the type of your division : infantry, para, etc...) in GameData\Gameplay\Decks\Generated\Divisions.ndf. There is already examples with our current division which retreive their texture from GameData\UI\Resources\Textures\Generated\DivisionTextures.ndf. To add a new texture you can copy/paste an existing texture, add "Modded" to the type (becoming TUIResourceTexture_CommonModded) and set the FileName to you texture (which need to be under AssetModding\). You can find a more explicit explanation on how to add a new texture in the documentation.
All unites are described in GameData\Gameplay\Gfx\Generated\UniteDescriptor.ndf, a list of all unites is available in GameData\Gameplay\Gfx\Generated\AllUnits.ndf
No, your mod needs all files to work, but you can remove data like divisions, unites, etc...
Any info on this documentation you mention? Is it something upcoming or something released?
I found your mentions to INTERFACE_OUTGAME.csv confusing as we don't have this file.
For anyone modding: all you have to do is fill in TOKEN (reference in game files) and REFTEXT (shown text)