Hi CiCi
I have spent far too much time on this but at least now I can provide some positive feedback, also for the benefit of others.
When I last wrote to you, I could create a MSPK but Earth Mobile would crash as the elevation source used was in raster format (tiff in Web Mercator). In Earth Desktop the imagery would load but not the elevation surface. It could not get the values from the tiff file, but in ArcGIS Pro the MSPK could be successfully loaded. I suspect this is an ArcGIS Runtime limitation. But by including an elevation TPKX file (created using the Manage Tile Cache GP tool) in a Web Mercator projection, the MSPK files would successfully load in Earth Mobile and Earth Desktop.
But probably my biggest headache is a bug that is present in ArcGIS Pro 2.6 Beta 3 and ArcGIS Pro 2.5.1 (the two platforms I tested). It is only after you told me that the TPK/TPKX files must be Web Mercator and in a Global Scene that I persisted.
My just (1) creating a Global Scene; (2) adding the TPK in Web Mercator; (3) adding the elevation source as a TPK file (also in Web Mercator), I would always get an error message "ERROR 002768: Package blah blah .tpk spatial reference incompatible with Mobile Scene Package" (see below image).

However, I finally figured out that it was a bug and that I needed to convert my Global Scene to a Local Scene, change the projection to Web Mercator, then convert the Local Scene back to a Global Scene and then I could create a MSPK.
If the imagery and elevation TPK files were in Web Mercator too, then it would also successfully load into Earth Mobile and Earth Desktop.
Unfortunately the Beta program closed on Friday. I will try and tag the @ArcGIS Runtime and @ArcGIS Pro teams.