(hey, still have some battery power left..)
A. How sustainable is the existence of OpenACS?
The software is forkable, customizable, sustained, maintained, and improved upon by a community of developers. Try that with proprietary software.
A snapshot of the project (taken at any time) could be viewed as a loose international joint-venture of individuals, consultants and companies with collectively overlapping goals.
For marketing purposes, a situation analysis should be made of the openacs system, before its strengths, weaknesses etc are identified and assessed for applicability to any purpose. Some of your questions seem more specific to the latter...