I write some notes here on to how to perform these features:

Preview

I hesitate if that could be accepted by all users as default mode otherwise create a checkbox to enable/disable it.

• with add mode, on area enter create the new cell immediately and switch to move (implicitly without showing the mode change in the selection)
• on click : stop the move
• on area exit : remove the new cell that stayed on the edge

Undo

Easy one :

• this.array.pop()
• also possibly if we keep the cell count in sync with the seeds slider, then moving it backward will be removing seeds in the creation order, but the slider will sample new random seed on increase
• an undo, redo slider could execute the history sequence in the right order

from voronoi.