Let me preface this by saying - I am NOT a nice person(in fact, i'm kind of a dick), however i am a fair and honest one.
1. I'd have taken the Space Invaders I had in stock, shown her the price and remained firm. I'd have blatantly let her know that she was mistaken, and if she insisted that she was told via phone. I'd simply have told her tough - This is what we pay. The problem I'd have had with this situation, is that i'd have known i was CLEARLY being railroaded and taken advantage of. Which don't fly with me. If that costs me a customer? So be it. Sometimes you have to think like that.
2. I'd have probably handled this situation the exact same way and taken the 10$ loss. The fact that she came back later and such would have led me to believe that she felt genuinely slighted, and I'd have just eaten the difference, while assuring my cashier in the way you did.
3. I'd have let them work out the trade, and sort of held my annoyance in. The problem with booting them out, or yelling at them is that you end up looking like a greedy prick no matter what. Visions of "The Comic Book Guy" from the Simpsons enter my head immediately. It's not as if the person trading the items in was soliciting them to others. Someone simply overheard, and offered him more for items he want. It's something that probably most of us would do. If i seen for example some guy trying to sell Q-Berts Qubes for the Colecovision(A game i currently want for my collection) for 2$ at some shoppe, i'd immediately pipe in with a better offer. I probably wouldn't give a shit if the shopkeeper cared either. More importantly i'd say: Is that the overall bad feelings you would generate by kicking them out or interfering with people who are obviously interested in the exact sort of product and merchandise that you sell, isn't worth the small gain on the trade-in's. By not interfering, they will be more inclined to come back and give you future business.