If you think that would provide useful results, nothing prevents you from trying to obtain it. What have you done? Oh, that's right -- absolutely nothing. I don't think you're motivated by the desire to save a lot of talking.
No. You don't get to argue based on how you think tests would turn out that you haven't actually done. Further, the rifle "leaking like a sieve" proves nothing if you can't answer the demonstrated unreliability of the paraffin test. The condition you aren't considering is the combination of (a) Oswald's Carcano leaking like a seive, and (b) the paraffin test on his cheek being a false negative.
The parsimonious answer for the hand tests being positive and the cheek test being negative is a leaky rifle and an unreliable test. You seem to think that in order to refute your claims, we need to prove the Carcano couldn't have deposited powder residue. That essentially presupposes the hand tests were both false positives and the cheek test was a true negative, and that a massive rifle switcheroo occurred at the hands of the Powers That Be. William of Occam is not pleased.