Is it possible to have a chest wall that caves in and protrudes but not have Marfan syndrome?

Yes. Normal people sometimes have a chestwall deformities. One should not worry about marfan syndrome unless one has several of the physical appearance features (then a doctor's evaluation should be obtained). Having just one of the physical appearance features in an otherwise normal individual does not necessarily suggest marfan syndrome or any other problem.
Yes. Most people with sternal abnormalities (pectus excavatum or pectus carinatum) do not have marfan syndrome. There are other diagnostic features for marfan syndrome that are much more important in making the diagnosis of marfan syndrome.