What is the state on Third Party parts?

[Eugen]

Years ago it was a joke that "LDraw.org opens to LEGO clone bricks":

• https://forums.ldraw.org/thread-16041-post-16046.html
  

Now I see there are some Third Party parts on PT, and some of them already certified:

• PT: Part Class: Third Part Parts
  

Third Party are not exact LEGO clones, but a LEGO-compatible parts with its own design.

So:

1. What is the state of Third Party parts?
   
2. May I design own LEGO-compatible part and pull it to Parts Tracker for including into Parts Library?
   
3. What is the state for including non-LEGO-compatible parts from other construction sets now?