I don't think grieving in itself should be labelled a mental illness.
It's a part of life, and, everyone does it different. There are some things that seem to happen in almost all grieving processes though. And, based on that, you can see when people get stuck in a grieving process, where it becomes a problem.
That can happen after two months, after a year, after 6 months. It should not be the time that dictates when a process has gone off in an unhealthy route. And people should not have to wait for a year till they can get appropriate help when it is going wrong for them.
If it gets one label, will that mean one way of help provided by the insurances? Grieving that turns problematic will be different depending where it went wrong.
Someone still in denial after a year needs different help than someone stuck in anger, or someone in a depression. The label makes no sense to me.