Comment, extending stem: This may not be a problem... or it may. Because of the limited number of frames, the planking may lay in. We do not recommend altering the stem without re-lofting the lines as we cannot predict the outcome. We would have increased spacing from the aft end of the stem to the transom and left the stem alone.
Comment, building form: Allyn has done something similar to this in the shop. His base was a plywood box and was completely rigid, with no wracking whatsoever. If this form does not wrack and with the other precautions mentioned, this is perfectly acceptable.
Comment, reinforcement: There is nothing wrong with additional reinforcement... up to a point. Any additional structure adds weight. I have talked to many builders who have carried this to extremes and ended with a boat that was much heavier than the design and therefore "sluggish" in performance.