It says in the core rules that dual natured creatures do Strenth M damage in astral combat. Could a shapeshifter choose to fight an insect spirit on the astral instead of physical if it is amaterilizated spirit form? what about flesh form? If they can choose to fight them in astral would their benefit from natural weaponary in astral form help? Also on an unrelated note what are the rules for regeneration of stum damage?

I'm not sure about the rest, but you can't regen stun damage.

Yes, you can definitely regenerate stun damage. That you could not was a hold over from previous editions that no longer exists in 3rd.

Regeneration heals "Damage" and that's pretty much it. Damage is both physical and stun, and it is never stated in 3rd edition that regeneration only works on one or another.

This does pop up a whole lot whenever anyone mentions regen.