With regards to #1: What happens is that if task_B depends on a value that task_A returns, task_A will insert its value into the queue and task_B will consume it.
If task_C returns a value which no other task cares about, it will insert the value into the queue, and never gets consumed. This is why dead queues (also known as "tombstones") happen.
Always remember to set ignore_result=True for tasks which don't return any consumed value.
In general using an AMQP for the result storage is somewhat of a bad idea i think. But yes I agree about the ignoring results part seeing as most tasks I've seen in the wild don't return anything at all. Hence #6 in the post.
If task_C returns a value which no other task cares about, it will insert the value into the queue, and never gets consumed. This is why dead queues (also known as "tombstones") happen.
Always remember to set ignore_result=True for tasks which don't return any consumed value.
EDIT: "Tombstones", not gravestones