Modified core races and the tech tree

You will have to build your new race from scratch. When you ran your race, you chose Yor and their default settings  are still there (which you can't edit). Just start the game over and choose the "custom race" from the player start menu and then you should be able to edit your custom race as you like.

Try going into the Yor and resetting their points/etc, saving them, then going back out and trying Custom again.

I tried what you suggested, and even deleted my race config files (including the Yor one). However, when I select the "Custom Race" (in the upper right corner), it again brings up the Yor Collective, disallowing my choice of another tech tree, history, special ability, or such.

I hope you only deleted files from the "My Games" folder. If not, you going to have to replace the file of the Yor you deleted. The default for the game is IIRC Human. So your game should default to that. Make sure that the game is set to "NORMAL" in the setup screen (below the map setting) and that your not playing a scenario or campaign...

I checked everything, even moved my saved games so TA wouldn't see 'em. But when I'm on a "normal" game, and I select "Custom Race" (on that selection screen it even shows "Custom Race" led by "Custom Leader" and from "Custom Homeworld,"), it shows instead the Yor Collective, led by N-1 from Iconia. The UI doesnt change from the Custom Race's cyan, but everything else is Yor.

I'm going to try a reinstall.

Okay, so reinstalling via Impulse didn't work either.

That's pretty odd. Are you sure you cleared the TA folder, not one of the others?

Finally got it to work correctly, by (re)moving the "Prefs" config file.