Have you tried RedCine X?
FCPXML with RedCine X can be weird, so I export an Fcpxml from FCPX, convert that to XML using XToCC. I import that XML in to RedCineX and it should relink to the RAW.
I then export an XML out of RedCine and import to Resolve. From Resolve I can go anywhere (EDL, FCPXML, XML, whatevs).
As far as the new shots, you can provide just the new media and a new XML and they should be able to conform it so that they keep all the work that has been done.