Hello andrey.d,
You second code part is not correct. You are changing the coordinate system of the chunk (not converting, but just changing the system name like you can do via Reference pane settings), but after that the orthomosaic is exported with the default coordinate system.
.exportOrthomosaic() function has projection argument, that you can use to export the result in a different coordinate system, without affecting already processed data.
So I can suggest to build orthomosaic in original chunk coordinate system and during the export use projection = crs argument, where crs = PhotoScan.CoordinateSystem("EPSG::3857").