The MRI results are probably the key now. A falling M-spike and IgG would usually be encouraging, but they don't always tell the whole story. The targeted MRIs should help determine whether those lesions are actually related to your plasma cell disorder or something else entirely. With a 9% marrow you're right on the MGUS/Smoldering Multiple Myeloma border, so I can understand why this feels confusing. Hopefully the follow-up imaging will put the blood results into context. Out of interest, has your calcium level been elevated at all, and how have your free light chains (kappa/lambda ratio) been? Those can sometimes provide additional clues alongside the MRI findings. With my wife they suspected MGUS, MM or SMM but when I checked the blood results her blood calcium levels were very normal so I asked them to check for LPL IgG as this was a better fit. So I am sure with the correct tests things will become clearer. The key is often hidden in the blood results marrow and imaging and your clinicians sound very competent. The type of cells in the marrow should also give them an indication of the underlying issue.
Jon