Gary,

I just upgraded to v27 last night, and I am not all that familiar with XML yet.

However, re: key & time signatures... be sure to "Edit filter..." before you copy,  and be sure to check EVERYTHING.

Even then, sometimes in v26 it would not copy the key & time signatures. If not, try it again.

Hope this helps,
:-)
Nessa

Also, to truly copy everything, use Stack Selection.

Stack Selection is also possible if you do not include all the staves in the selection.

But it requires that you create a custom staff set of only the staves for the selection.

Then, while viewing that custom staff set, make the Stack Selection.

You don’t mention the source of the .xml file nor its version. MusicXML is a toolkit and how well it works depends on its implementation by an app’s developers. Transposing instruments not behaving properly is common, unfortunately, but there are workarounds. If Finale to Finale, this can usually be fixed in Preferences. Sibelius to Finale is easier with the correct Dolet plug-in and so on…

You may at times run into font issues, too, when you try to copy and paste directly from older Finale files in to 27.

Good to know.
:-)
Nessa