Question: How does Don Self know that Miriam should work at Gate when nobody knew about the location of Scylla? Or Whistler wanting to work there?
Answer: Because Miriam was an agent.
But that still doesn't make sense because nobody knew where Scylla was.
He knew that GATE was mentioned in the bird book. He worked with Whistler and had probably discussed GATE with him, so it was clear enough that it was a big part of the plan to get Scylla. Sending an agent to work there in any position would've been a big help for intel, as she could assess the environment and perhaps uncover what Whistler's plan was. He didn't anticipate T-bag's hijacking of the plan, hence why he sent Miriam and not a more combat-trained agent. She was only there for information.
Answer: Don Self actually worked with Whistler before he died. And Whistler with the bird book actually knew where Scylla was.