I won't comment much on the two last replies (especially the last one, just please don't bother with such useless answers in the future), but as for the first one. . . How would I do that? This whole "do a DB edit" is the core of my question, I don't know how this database looks like, what is there etc etc, so how the hell am I supposed to figure out which "db edit" I'm supposed to do, not to mention how to perform it? Otherwise thanks, changing the npr to a player race seems like a good point if sm allows easy deletion of it, much simpler than "deleting dozens of entries".
Also, to clarify: of course I can do backup, my fear of screwing things up isn't about blowing my save up in obvious ways but rather small but significant errors that might resurface at some point after I'll invest more hours into my save.
Disclaimer: I have never done this before and make no guarantee that it will work,
but I see no reason why it would not after poking around in the DB to figure it out.
Open the Aurora DB with DB Browser or any other SQL tool of your choice. Find the table "FCT_Race". Scroll to find the row with the race you want to get rid of (if you're not sure which this is you'd have to muck about in some other tables and cross-reference with your racial intelligence in-game; in this case, you just want to blanket-delete the non-spoiler NPRs so it should not be a problem). In the column "NPR" change the value '1' to '0'.
This
should allow you to select that race in-game from any of the racial drop-downs. In-game, select that race in the Race window and click the Delete Race button (you might need SM Mode active for this).
DO NOT try to delete the race in the SQL editor, as this will break references to that race in any other tables (ships, fleets, research, colonies...) and probably break your save. Delete the race in-game so it will be handled correctly.
Be careful not to edit or delete the
Precursors, Invaders, or any Swarm races (plus Rakhas in 1.13+ once they are fixed) as these races must be present in the table or else you will break the spoiler races.
If it is possible to set a race as a player race in-game, I do not know how. I do know that it is not possible to set a player race as an NPR in-game due to AI problems, but the reverse is supposedly possible. In any case, the DB solution does (or should) exist.
EDIT:
A couple pointers that might help - FCT_AlienRace sounds like it might be useful, any of the FCT tables that have "Alien" in their name are probably related to an NPR. You can also search for "RaceID" and find the actual ID of the NPR race which could also help find other relevant tables.
The "FCT_Alien..." tables usually are related to intel data, i.e. information each race has about other races. The "Alien" in the name refers to the fact that the data is about one race's knowledge of another.