There is no load filter or non-default loading in EDCE. if there is loading that you do not want to occur, you should use the tree view.
Air Units and Orbital Units cannot be placed outside of transports (cities, etc) in a buy points game. Can Be Placed is also used for other actions. The Unit Mod capability is simply an exposure of the available data. Many entries may have hidden or non-intuitive results.
The load and unload buttons have been disabled but will most like be enabled in a future build.
When in the tree view, clicking a unit does not activate it, because multiple units can be selected at a time. If you wish to activate a unit in the tree view, select the single unit, then click on the activate button.
If you are referring to the save config from the game setup, it only saves a Database key. The game will assume you are aware of the changes you made in a DB. The same is applied to the saving of a single player game. You can exit a game, change the Unit set, then reload the game. This could have ill effects on the save game in question after a reload, but it is up to you to police this.