Lambert death works best as it is in the film.
It always felt to me that after all the other headbites and everything else we saw, her death was so violent and the state of her corpse so repulsive, that it couldn't even be shown. It made the moment that much more hideous.
Why the lack of shoes and such was always debatable. It could be since she was female, the alien did something sexual. I always felt it just tore into her with such force and violence that it basically had the same effect as a car wreck. I think she was utterly decimated.
I think her uncontrollable level of fear was intoxicating for it, and it got off on it, hence it toying with her. I think it reveled in scaring her to death and then just unloaded on her in a full blown rage-gasm. Heckfire, the dang thing roared.