Do you think you could show a screenshot of the agent's heights in the scene view (should be visible as yellow cylinders).
As for the second issue. That's because it is overshooting, it tries to move to a point on the path, but the FPS is too low so it moves further than it intended and ends up on the other side of the path, at which point it tries to move in the other direction etc. The effect is exaggerated by RVO which introduces a 1-frame delay in the movement (make sure you have double buffering disabled as that otherwise increases the delay to 2 frames). This is usually to be expected if you have a very low FPS.
Are you using the AIPath script with the beta version? If so, you can fix the issue by opening the AIPath.cs script and adding this line
velocity2D = Vector2.ClampMagnitude(velocity2D, pickNextWaypointDist / (Time.deltaTime + 0.0001f));
right before the call to
(around line 254).
This will prevent it from moving too quickly (I think it should work anyway), however it will result in that if you have a very low FPS, then your characters will be forced to move slower than they normally would.