I am so sorry to hear this. Not knowing what happened is an awful feeling, losing a baby bird is even worse. I am glad for your other birds that the tests came back negative, although I know that doesn't bring much closure.
Finchly was very kind to take Twig back home when Cricket died. I had planned to adopt another baby if the worst happened, but when it did I was so irrationally scared that the same thing would happen again, especially because the necropsy was inconclusive.
Attention from you will do more to comfort a budgie than a finch, so I don't think you should rush yourself. Think about your own feelings, too.