That would be the correct image, it will flash to the other partition, i.e. the one that is not current, then you could try scp the OEM to /tmp and ssh in to try the above CLI sysupgrade line. Which will overwrite the other partition, i.e. the one that is current, you can then flash via OEM whatever it is you want on the other partition and restore your config backup, leaving you with OEM on the inactive partition, and (assuming you flash the same LEDE as current) an image that apparently you cannot get off of, on the active partition; so back from whence you came. But, will answer(maybe) the question as to why you cannot get from either??? LEDE image to OEM.