Choanoflagellates and metazoa also have flat christae in their flagellar mitochondria while protista have tubular ones. In fact I could be describing the human sperm. Virtually all metazoan cells except eggs and sperm are diploid. There are so few exceptions that it seems likely that the early metazoa were diploid with meiosis preceding the formation of eggs and sperm. Metazoan eggs develop from one of the four products of meiosis, the other three forming polar bodies and degenerating.
Metazoans also have collagen.
Similar glycoproteins containing hyroxyproleine and hydroxylysine are found in plants, animals and fungi but those in plants are extensins, and those in fungi are not collagen. Again that makes Volvox an improbable ancestor. Collagen lives between cells and in basement membranes or more elaborate connective tissues such as our skeleton or an insect cuticle. It is found in sponges, but no one seems to have looked for it in choanoflagellates. The next important question is this.