How do you perform hydrostatic calcs without atleast a set of offsets? How do you know where it will float (draft and trim)? Boat building is an expensive project to undertake without knowing that the end result will float properly.

There are some good, free programs about that take care of this. An accurate weight estimate (including centre of gravity) and some simple hydrostatics go a long way towards achieving a desirable outcome and minimising the risk.

A couple of frame sections combined with your foam fairing is a possible method of mould construction. Although you should look at what the professionals do, they will always choose a commercially sensible option. Generally speaking, a set of mdf frames will be cut and stood up then covered with a skin (mdf or strip planked).
Building yacht