Should I take threats from a former professor seriously?
This sounds terrible, and like you said, I definitely think asking her to write a letter of rec was a mistake.  However, regardless of whether she tried to sabotage you or not, you've been admitted!  Congratulations.

Now is the time to sever all contact with her and move on.

You left a record with your previous university about your issues with her, and if she was going to create problems at your new school I really think it would have come before you were admitted, especially since she was a letter writer.  Whatever she did or did not say, they gave you a big vote of confidence in offering you a place.  Time to focus on the next stage.

Good luck!