Does replacing onlays require even more removal of tooth structure?

Not necessarily. Many of my colleagues may disagree with me. While that is often the case & may be necessary, for any number of reasons, it does not have to be the case. It boils down to why the onlay needs to be replaced. There may be recurrent decay, fracture, insufficient depth or design, or an occlusal problem that requires further tooth removal. If it's just a matter of shade or wear, not the case.
Yes. Anytime a dentist is replacing a restoration, some amount of tooth structure will be removed. It may not be much, but just the simple process of removing something old will lead to loss of tooth structure.
Maybe. The design of a dental restoration takes into account the missing tooth structure due to decay or fracture, and the preparation of the tooth to retain the restoration. Depending on why the onlay failed, your dentist may have to remove more tooth to make the next one more retentive.
Yes. . Yes, onlays require that a small amount of enamel be removed from your teeth in order to accommodate the porcelain shell or composite material so as not to add bulk. Because of this, they are not reversible.