Thanks for the help and links so far! Allright, it's definitely a threaded headset, original Mongoose stamp on the top nut too -
The top hex bolt on the neck that goes through the tube is a bit stripped. I'm gonna soak it with BreakAway and then try to get it apart. Once it's apart, I'll repace that bolt. The other 4 bolts that hold the handlebars in place are in good shape.
And it turns out the crank is actually a 1-piece GT Performer setup. I ~assumed~ it was a two piece because the outside ring is separate. Here's a couple pics -
