The two Davis shells were linked, through ballistics, to Oswald's revolver to the exclusion of any other weapon. The chain of possession for these two shells is clear and perfectly intact.
"Oswald's revolver". LOL.
I think you mean the revolver that Gerald Hill pulled out of his pocket at the station two hours later which only then was initialed by any of the cops.
Any issue (if there are any) with the chain of possession of the two Poe shells does not change the fact that the two Davis shells convict Oswald.
What is your evidence that the shells that the Davis women handed to cops came from the boy they saw crossing their yard?
Or indeed even came from the crime scene at all?