I have heard of others losing a young bird out of the blue. My Amazon did nearly the same thing, fell over face first in his food bowl and out like a light, but he was around 15. It was also 20+ years ago when I got him and no one knew what the heck we were doing especially in the feeding department.
It's very tough losing a bird and then dealing with the guilty feelings of wanting another. You're not disrepecting the memory of your passed-on little friend. They make such a mark on you that really it would be highly unusual for you not to want to fill that void again, although you can never replace them.
There are several around here that have gone through some heart wrenching losses including myself. Recently Muse just lost one of her birds out of a beautiful pair. She has taken in another that would most likely end up who-knows-where but is better off for it now. We lost a sick little Jardines that I was highly attached to and I never thought I would get over it.
As for food - I wouldn't change much of what you were doing, especially until we hear back from the vet report. I see nothing wrong with what you were feeding. Supposedly Harrison's is the best of the best, it's what we feed outside of fresh veggies/fruits and sprouts. The ingredients list is very different than most.