First, thank you so much Recoleta. You've given great advice, you are very wiseISTJ's hate disappointing people, and like even less hurting those they are close to. With ISTJs, you need to be very direct and unwavering. Don't be afraid of hurting their feelings or stepping on their toes...they'll get over it.
However, I am nervous that if laying it all on the table makes them feel like they are somehow treating us unfairly, wouldn't they want to walk away seeing as how they do hate disappointing people? It's like, I'm nervous if I say the wrong thing and they see exactly what they're doing, they will feel so bad and say "See! That's why I didn't want to do this!"