If I get you right, your issue is that you cant spawn objects in Cells that arent currently loaded?
SetPosition X/Y/Z isnt working because each Worldspace has their own X/Y/Z coordinates. Meaning a Coordinate 42/13/17 in Tamriel is not equal to the Position 42/13/17 in Blackreach etc
Does your mod care if the objects are there right away or na?
Cant you just save the specific Cell ID in the json and check if the cell ID is present everytime the player changes a cell and if so, place the objects when the player loads the cell?