Unintentionally started installing the TP and now I can't cancel the DL. Only thing
I can do is pause it, which won't let me launch the game.

And for that matter, once I do finish DL'ing, how do I get rid of it if I don't like it?

You can't. It's registered as DLC and works like a game update.

You deactivate it in the launcher. You can't remove it from the HDD without going offline and staying offline.