LR can definitely leave the image files themselves in the filesystem, and not pull them into the database. For the metadata, it's not quite so seamless, but it helps to select "always write metadata to XMP" in LR's preferences. I don't know how seamlessly it'll notice modifications to the XMP metadata made by another app (say PM), but most changes made in LR will be reflected in the XMP metadata where PM can see them, and not just in LR's private database.
LR is supposed to have plugins coming someday that might help with a lot of integration tasks, but they have yet to materialize.