I am so so sorry for your loss... especially with just being a baby and such a brief time in your home.
Not meaning to sound heartless, but with being involved in rescue world, disease testing is a big concern for a situation like this... Did the shelter do disease testing, specifically including Pacheco's? That is a worry with a very young bird... With worry about your other feathered babies, you can have disease testing done fairly quickly thru avianbiotech.com. With being an urgent situation, pick up a new box of qtips and ziploc baggies. You can do your own sample kit and overnight with form you download from their website and have answers for the major concerns of pscittacosis and pacheco's within days. Submit all the samples as environmental / poop samples. PBFD, pscittacosis, polyoma and Pacheco's can all be run as environmental.
If you have any questions, I am happy to help you with the testing process, you would have disease test results back much quicker than a full necropsy which can take weeks...
Again, I am so sorry.