There are people who are allergic to iodine which is why the hospital staff always asks if the patient is allergic to it before they use it. The way you describe her final moments, it sounds similar to a severe allergic reaction but it may also have been related to her heart, perhaps a massive heart attack. The autopsy can find out exactly what happened.
I'm sorry this happened. Such a shock and not exactly the final moments you would wish for your mom. Perhaps some explanation of what went on will give you some peace of mind.
It sounds to me like an independent autopsy would be the exact best way to establish a cause of death. I am so sorry for your loss. It is just terrible to lose a loved one under any circumstances, and especially like this. Please accept our deepest condolences. Hopefully, you will know more after you get the autopsy report.