I would not wait til it was rancid. I would offer it in the mornings as a wet mash.
A wet mash is simply feed with enough water added to make a cooked oatmeal consistency mash.
If they still refused to eat it and I had no one else that wanted it I would throw it out.
In a compost pile mice may...
My understanding is that the nutrients begin to degrade as of the mill date and by 6 months are degraded enough to be offering reduced nutrition for the birds.
I don't know if that applies to whole grains in the same way it applies to things like pellets and crumble.
I agree getting the barrel...