I am working on adding phpbb to fgbmfi.net and I am working on the exact same problem.
Currently I am using "Google XML Sitemaps with Multisite support"
With my own custom PHP script. It would work for single sites but that plugin is not very well maintained. I am trying to modify my script for the version 4.0 beta of the plugin you are using.
I wonder how I can include the tree of pages that belong to phpbb forum into this sitemap?
Where will you get this "tree of pages"? If you can tell me, I may be able add this, and help both you and me.
I wonder if you have looked at:
Better WordPress Google XML Sitemaps (with sitemapindex and Multi-site support)
This looks more active, and has the options to add extra sitemaps (like for phpbb) and it looks like it was designed to be more efficient for large sites. We all want a large site, right?
As to your question about Google Webmaster. I submitted the .gz version of my sitemap to Google.
I allow the plugin to also ping Google. For me this submits a slightly different URL. Anyway the results were like this.
The sitemap I submitted via Webmaster was indexed first. Within 3 days for my whole site.
However, the updates that the ping submits are indexed withing about an hour. So my site is pretty much up to date, within an hour of any change. I, like you, thought to remove the Webmaster entry. But then I had a bone infection, had a surgery and basically everything came to a screeching halt for about a month. While I was away Google came to my site once a week and checked my Sitemap, I believe because of the Webmaster entry.
My advice, both submit and ping! It works for me!