Well thought out post. Thank you.
Re. the portion I bolded - I've wondered the same but considering they came back later to get samples of the dog's DNA, this has been my line of thinking:
Right after TS's body was discovered and LE processed the crime scene, they collected DNA samples from all over the house, and probably including her body too. They sent those samples to the lab for processing. IIRC, it was several weeks later when LE came back to retrieve the dog DNA specimens. I was thinking that maybe the lab contacted LE to tell them that while processing the first samples they discovered a mixture that contained non-human DNA. If that were the case, they would probably need the dog's DNA in order for the lab to isolate the "mixed" sample. Does that make any sense? I'm no scientist for sure but I've followed enough cases on here to know that might be a possibility.
In fact, we have a pathologist who is a member. She posted extensively in Caylee Anthony's case. Tomorrow I will contact her and she is she can shed some light on what that may mean. It's been bugging me for a while.