This thread was great to read. We're just beginning this journey so I can really only empathize. My 2yo is delayed and we are still searching for a diagnosis. My older kids are average to accelerated depending on the subject. Right now they all just think of him as a baby, so they don't expect much from him. His receptive language is on par with a 12mo, so he doesn't really understand much they say yet anyway. But I imagine there will be questions on both ends eventually.
One thing I have really been focusing on with the older boys is having a "growth mindset." I try to praise their effort, not their accomplishments. I tell them the important thing is how you go about getting an answer, not the actual answer. We talk about how we work hard so that we can grow our minds (and we go to church to grow our hearts, eat healthy to grow our bodies, etc).