A protocol for encountering an entity you didn't believe in has also been established:

"This is a child!" Haigha replied eagerly, coming in front of Alice to introduce her, and spreading out both his hands towards her in an Anglo-Saxon attitude. "We only found it to-day. It's as large as life, and twice as natural!"

"I always thought they were fabulous monsters!" said the Unicorn. "Is it alive?"

"It can talk," said Haigha, solemnly.

The Unicorn looked dreamily at Alice, and said "Talk, child."

Alice could not help her lips curing up into a smile as she began: "Do you know, I always thought Unicorns were fabulous monsters, too! I never saw one alive before!"

"Well, now that we have seen each other,' said the Unicorn, `if you'll believe in me, I'll believe in you. Is that a bargain?"

-- "Through the Looking Glass", ch. 7, Lewis Carroll

a girl I dated once claimed that she imagined herself becoming an atheist, imagined God's subsequent disappointment in her, and this convinced her somehow of the existence of God

Wouldn't this reasoning apply to any other deity that would be disappointed in her disbelief? She must believe in an infinite number of other deities as well.

I think the idea that one human not believing in the existence of another is in some way rude or disrespectful has already been somewhat established

Homer: You monster! You don't exist!
Ray Magini: Hey! Nobody calls me a monster and questions my existence!

That's a great story, but I don't buy your interpretation. I'm not sure what to make of it, but it sounds more like a vanilla Pascal's wager.