To answer your questions, no, no, no, and no.
There are no unused assets, no unused XML entries, no way to avoid doing huge amounts of work to get new unit graphics.
If you want to add a new unit, you need to create a 3d model, with its own textures, or import these from another game. For instance, I've added quite a few unit models to my own mod, using unit models from Civ4.
3d models animate using keyframe (.kf) files that tell a skeleton how to move. Because of that, it's possible to use one model's keyframe files for a slightly different model; as long as they're still humanoid and have somewhat similar types of animation, it can be done. But it has to be done inside Blender, with quite a few conversion steps after; Civ5 can't just accept a straight .kf file as an input.
The above steps can be shared freely between modders. That's what the Unit Graphics forum is for; everything up to this point has been done for you, as long as you only need one of the units someone has already made.
Then, you have to add those models to the ArtDefines files. THAT is not only not trivial, but it also is completely unique to your mod. You can't take a mod that adds one unit, and load it at the same time as a mod that adds a different unit, because both mods have to replace several files IN THEIR ENTIRETY. It's not modular.