I don't know if I can answer any of your questions directly, but I can relate a bit of my experience and why I implemented a community site for a weekly business publication with OpenACS.
I will address one question directly, "will ACS keep up?". I don't believe this is a valid question as OpenACS and Zope are vastly different beasts. Zope is an application server that, to me, does provide a good environment for building database-backed websites. However, I do believe that the volume of products listed on the Zope site is somewhat misleading. When I looked at Zope 12 months ago, most of the "products" were very tightly focused utility type programs as opposed to fully functional products.
OpenACS is a pre-built application for implementing community-based websites. Out of the box, it provides 90 % of the functionality that I needed to implement the site for the news publication. The only major modification that I made was to implement an edition/time-based model for the the news module. I haven't looked at Zope in 12 months, so I can't say what it can do now, but at the time I would have been building most of the functionality of the website from scratch.
Also, as I recall, Zope was no easier to install and get running than OpenACS. And remember, the truly valid metric is not how quickly and easily the base platform/tool is to set-up, but rather how soon do you have a production site that meets your requirements.
