The stiffest material in a composite is the wood. The wood gives the bow its shape. That is why you should use a thick core to start with. Jeffery's video is useful but he makes many 'mistakes'. I understand that he figured out a personal method but the old guys knew exactly what they were doing and every single step has solid reasoning behind it. When I first started making composites I had to stop myself from experimenting with different ways to do things....Follow the old methods 100%. If you have Adams book just disengage the brain and follow exactly what he says.
The shape is a combination of the shape of the steam bent core pre horn glue-up. Then the width and thickness tapers you use once you are shaping the core/horn for sinewing.If your bow doesn't bend in the correct way when first braced you will have a hard time.
Those clamp on form thingies are called teplics and the reason for their use is to be able to bend the limbs to a position close to brace without having to actually brace the bow. They will show up twist pre-bracing. They are also very useful for bracing the bow the first time so you aren't fighting all the reflex when the bow could still be twisted.
Heat can be used to correct twist and the curvature of the kasan eyes (Turkish bows) but should NOT be used on the sals when the bow is strung.
I'd strongly recommend using Adams book!