Flush with the finished floor is optimal, so it looks OK in that regard.
The warp is a bit concerning, it looks like someone forced it down over a bump or protrusion or maybe over tightened the toilet to it and damaged it (although it would be expected that the toilet itself would break under that amount of stress).
Leaking issues, toilet rocking back and forth
These things point to a loose flange, loose toilet attachment, or lame application of wax seal. Have you grabbed the flange and pulled on it a bit to ensure it is attached to the pipe and floor properly? I can't see any fasteners in any of the many available bolt holes provided to secure the flange to the floor (although it may be the angle of your pic).
I try to use only cast iron flanges because I have seen many plastic and thin steel ones break. Maybe it's time to call a reputable licensed local plumber (instead of the builder or his "plumber") and bill the builder for whatever repairs the plumber ends up making.