How best to import a new xml file?

Well, no, there's only one way to import an xml file. And indeed it is to be expected that not every layout detail survives the transition.

As mentioned, it's normal that formatting details don't survive MusicXML export/import - the notes themselves come through, and so basic layout things like system breaks, but aside from that, MuseScore does it's own layout.

As it it, there are some strange things with this file I don't really understand - like why most elements are showing as invisible. Also some of the formatting adjustments that did make it through don't really work, which is also to be expected. First step after any MusicXML import should normally be Ctrl+A to select all then Ctrl+R to reset manual adjustments. In your case, I'd also click "Visible" in the Inspector to make everything visible, unless it really was your intent for most of those notes to be invisible.

But then, there are other strange problems I've never seen before. Like, I have no idea why the accidentals on the first system are colliding with notes. This even survives copy/paste to a new score. Maybe someone else more familiar with MusicXML can find some clues?