In reconstructions, how are various shapes of facial features determined from skull only?
Disclaimer: I am an evolutionary geneticist. I know nothing about anatomy and forensic methods....

You should just have a look at wikipedia > forensic facial recognition. There are a number of different facial reconstruction methods, one of which is the so-called Manchester method (see a review of the Machester method in Wilkinson 2010).

In any method of facial reconstruction, there is some part of artistic representation to fill the gap of what we fail to infer from the tissues available. It is typically not so uncommon esp. for recent(-ish) fossils to have remains of soft tissues as well. In absence of such soft tissue, the morphology of the mouth and ears are typically left to artistic interpretation.