Dear Cici
Thank you for the feedback. I notice now that I had confused the issue a bit with mention of SLPKs when I was thinking of MSPK files. Sorry about that.
As I had previously received an error message when trying to add a Web Mercator tpk file to my Global Scene and then creating the MSPK, such as the below example. This error message made me believe that it was simply not possible to add a Web Mercator file to a Global Scene. However I did manage to eventually successfully create a MSPK with my satellite image as a TPK in Web Mercator.

This MSPK now loads in Earth Desktop however the Earth Mobile app crashes as soon as I add the MSPK file. I think the fault may lie with the elevation surface used in the MSPK. Although I have used a tif file as my elevation surface, with a known projection (and I have tried a few different projections), when I add the MSPK to Earth Desktop, I get an error that the file used as my elevation surface does not have a projection, when it does. See below error message. So in Earth Desktop, the scene loads but is flat with no elevation (when disconnected).
I can successfully load the MSPK in ArcGIS Pro and then I get no error messages and the MSPK layers are properly draped over the elevation surface. Perhaps this is an issue with ArcGIS Runtime and raster elevation sources? I will need to explore different formats for the elevation surface, unless you can offer some advice? I suspect I need an elevation tile cache.

Thank you so much for your help.
Best wishes
Mervyn