Hi Ed,
SourceIDs can’t be set by users. They are generated by the system and meant to be globally unique. It is the expected behavior that importing sources into a different tenant will generate a new GUID for their ID.
Any references to sourceIds in an SP config export will be ignored on import. The import operation will use the source name, not the ID, to make sure the import matches up with the correct source in the target system.
As for your issue with importing identity profiles, this is a known issue being tracked by engineering. There is another forum post regarding this issue, which you can follow here: Error importing identity profile - #10 by colin_mckibben. I will update that post once the fix is in production.