Question 1:

(When using an inner stem, should I plane all the end boards down flat to the apex of the inner stem or try to shape it into a rough point?:)

Use a belt sander and smooth the end grain of your strips at 90 degrees to the inner stem faithfully following the curve of the stem.Do not over do it, stop when you sand down to the inner stem.If you go further into the inner stem your outer stem will be much wider.If you try to whittle the end to a point i will look like a boy scout spear and not a nice one either.

(With an inner stem, the hull shear line finishes leaving part of the inner stem wrapped around the stem form piece. Should I carefully saw this piece off and save it for when the deck starts? It doesn't seem like it would be too easy to remove otherwise.)

Saw off the piece of inner stem and reattach it to the form for the deck you need the width to be the same as the hull stem.