Not sure where you live but I know in the UK I think it is the body can be kept for a long time with regard to a crime, I have trouble understanding that as it has to be further trauma for the family. That is no knock to them, just to say it is done differently.
I am no expert but from what I understand it does not take days for the actual physical autopsy of the full body even with suspicion of a crime in the US in most cases.
That however, is the physical autopsy I guess is what I would describe it as in my layperson's words. My best guess from what I understand is they photograph, x-ray, perhaps scan, take all samples and slides they need, pictures, etc. and then release the body and yes, the funeral is often not long after a body is found and the body can be released to family fairly quickly. I would prefer not to get more graphic but it is not uncommon from what I have seen.
It is the autopsy report, findings, various test results and review of what they take I believe that can take so long but they can take those things, have their samples and more to study or test and yet release the physical body itself fairly quickly (in most cases) for the family to lay to rest.
I think of it like we do when you go to the doctor--they will take your blood and urine for labs, they may do an x-ray or CT scan, etc. You can then go home the same day quite often while they await results which are then reviewed, the pictures, the lab results, etc., maybe send some things out for further testing, and then call you with your results--they no longer need "you" there. Again, I am trying to stay away from being graphic but they can take more from a deceased person internally than a living person. Once they have what they need, they do not need your body's "presence" to come to a "finding" I guess are the only words I can come up with without getting more graphic.
I follow (not closely but I do check it every few weeks) the Libby Squire case and it is remarkable to me the body is kept and the family cannot bury her--it is apparently very much the opposite in that country versus at least what I have seen in the US. However, again, I am no expert and this is just based on what I have seen or understand. If I understand from following the Squire case, it is due to the legal system mostly that it is different.
So having Savannah's remains with the family this quickly I do not find unusual, imho.
Anyone can feel free to correct me as I could be incorrect. This is jmo based on what I have seen.