Hi Paul,
There are some things that I can suggest; if these aren't enough, then our best approach is to work through Esri Support because they're really good at figuring these things out.
Since you mention the Global Account in the proxy config file, I take it that you're using the version of Maps and Apps that has the checklist.html page. I wrote the checklist to help diagnose the configuration; because it uses the same code as the app, it behaves the same as the app but provides the specific location where issues are found. But it does not try to upload anything to the account. Does the checklist finish?
The Global Account in the proxy config file should be the owner of the gallery and so should have the ability to add items. An uploaded file is added to the owner's content; from there, the owner can decide to share it with the gallery group.
Thank you for pointing out that Firefox no longer supports the 'click' event on the 'Most Recent' carousel. I can reproduce it in Firefox 20 as well, but I have not yet determined what has changed.
The RSS feed is provided by the Dojo 1.6.1 dojox.widget.ExpandableFeedPortlet widget. It's configured to show a maximum of 20 results, but only shows 5 as you pointed out. I don't see an option to offer paging or scrolling.
Mike