As you said, the LongListSelector is still based around GetEnumerator even for when rendering IList types. Most likely the control would need to be rewritten to support the purpose. While blogs might say it support it, none of them says how, so I'm not inclined to believe them. The LongListSelector's default is to render all items.
As for the loading of the data, it can be done progressive using a ObservableCollection. I got a example project that shows how to use a ObservableCollection in combination with the LongListSelector.
Basically this could allow you to progressively add more groups and/or, more data to the groups, and the UI should update accordingly.
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