I did read your long post with links, but none of the papers seem to be ruling out nuMSM neutrinos. Specifically, ~2-10keV sterile right-handed neutrinos with very small Yukawa coupling to active neutrinos. They do decay into active neutrinos, but lifetime is many orders of magnitude larger than age of the Universe. The resulting X-ray line is not expected to be easily seen. Tentative detection of 3.5 keV line might be it, though.