Donna Cappo sends the following official response to my third question:
QUESTION 3: Does ACM or SIGACT claim rights to the name “Symposium on Computational Geometry”? Specifically, if the SOCG community votes to end its relationship with ACM and organize an independent conference, will ACM or SIGACT insist that the independent conference have a different name, or that proceedings of the conference are published under a different title?
ACM considers sponsored and co-sponsored events an important asset of the association. The rights to an event, including the name, are the property of the ACM. Should the leaders of a meeting series decide to sever the ACM affiliation, ACM retains the conference/event name primarily to avoid confusion between ACM and non-ACM events.
As expected, Donna’s answer is entirely consistent with the “Statement of Understanding” in ACM’s conference manual.