I am not sure about (1) but you will likely need to create your own metabox from scratch and replace native one with it. See add_meta_box() in Codex for starters.
(2) to me seems to me as working against the system a bit. There is no point in hierarchy if your post belongs to whole hierarchy tree. On other hand this really needs to be put in context of how you will need to retrieve the data.
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