I believe what it means by "...with the ability to choose from among all furnishings available in the estate." Is that you have the ability to craft, and place any item that can be placed within the interior of the estate - not that it will allow you to use items already on the estate.

The Housing and Personal Room inventories are not shared. Imagine that, for an FC that had the full 512 members, that would be insane to manage.

50 items is per room, independent of how many items are placed on the estate already.

I am not sure how this effects the items that are "one per estate" though.