Bit late to help Xida, but others may benefit from an answer to this question.
To adjust linearity in the Geometry grid, use the 'mirror adjustments' check box to the right of the numerical value of the adjustment - see page 162 in the version 5 user guide. It's not much good if you need to use the full grid, but quite helpful otherwise.
If you're working with an off-axis projector on a flat surface, then stick with Perspective, Horizontal, or Vertical geometry, it will be a lot quicker, and should give you enough points. If your projection surface is complex, then you'll need to use a Full grid, and probably need to do a lot more than linearity.
As far as "undo" goes, it would be helpful, but no, it doesn't exist. A workaround you can use is to save (ok) the geometry window once you have it roughly the right shape, and on complex warps save each time you get one section right. That way if you do stuff it up, you can go back to the last time you saved.
Hope this helps
