I am sorry to hear you are worried about your cat's titers for Corona. My name is Rebecca. I am a veterinarian and will do my best to help.
Your requested expert appears not to be online; can I be of help?
As you know, having a titer to Corona virus simply means she was exposed to a Corona virus, and made antibodies to the virus. I am glad you understand this does not mean she has FIP.
There are different types of antibodies. IgM is the antibody that is made immediately in response to a virus or other pathogen, giving quick but not long lasting immunity. Then IgG antibodies are made; they take longer to make, but last much longer. So if the IgM is low, that is not unusual; IgM wanes early on and the IgG continues working.
Her IgG titer is also going down, meaning that she is not being continually exposed to Corona virus.
It is odd it says IgM canine. There is a canine Corona virus; if they measured for those antibodies you are not getting an IgM for feline Corona virus.
Let me know if this helps, and what other questions I can answer. The immune system is complex and complicated!