Hi there,
the core seams are missing from the filters UI because they are annoying to deal with. Every core chunk type is actually 2 resource types (open and sealed), and having them all in the filter list would clutter the UI considerably. In the future I may add an option to how them, though that will have to wait (as I would like to complete some more common features first and also I have to play Space Age for now).
Manually merging sites is on my list, it's just not as straight forward to implement.
Thanks for your message!