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"Would point-loading the hull at the keel cause the shroud tension to banana the hull? " i don't think adding a compression post would change the loads THAT radically. Banana problems come from too much tension, regardless of the mast arrangement.
Remember that a boat is a moving living thing, ideally you need to slack off all rigging (fore/aft/and shrouds) and let the deck RISE as much as it can. Then install the compression post, figure out a way to keep it plumb in column (by attaching it to bulkheads, or using collars/seats at both ends, etc.) and then...ideally you need to make the mast long enough so it is also firmly compressed and held under tension when the rigging is properly all tensioned. If the post is too long, it will be creating uneven stresses. Too short, and it may bounce out if the deck rises (dropping from a wave) or just not provide as much support a it could.
Unless your boat is known to have deck problems, you might just leave out the post and trust the builder to have built the deck well enough.
On post materials...almost anything can do for that much boat. A plain 2x4 in compression can hold up something like 1-2 tons, so a post or beam selected from good clear wood, even pine or redwood, would work. As would an aluminum pipe. I've never seen a stainless compression post, seems like a waste of money to me. But whatever you use, you need to make sure the base of it will stay dry, or be waterproofed, and that it is held in place, plumb and unable to bounce out. Probably means some type of collar at both ends, and those would also spread you loads, at least for the upper one.
Or are you prepping the boat for some typhoon sailing?