This is not a court case. Showing that someone can do it does not prove Oswald did it. Showing nobody can do it proves Oswald did not.
Why the push back?
And what would be the reason that the general standard should change? I find it odd that there would not be agreement, or even enthusiasm, for exploring ideas that are significant in providing evidence Trying to pin the onus on one side seems more of a polemic than a joint effort towards evidence
Further it seems like a dangerous precedent, especially in the assassination of a President, that we would lower the standard simply because a suspect is killed One cannot easily deny that an assassination where the prime suspect is killed should be a potential cause to create suspicion instead of time to relax To lower the the threshold of certainly in such cases should indeed encourage those governments, or elites of a given society, to utilize such a strategy since, as you suggest, the standard of evidence should be lowered, or as you imply reversed for proving any given suspect guilty
Maybe you might want to consider this argument in terms of a third world country Would you really want to suggest it is the interest of such societies to allow assassination subjects to be killed, and then to lower the standard of evidence tha tthe given suspected killer was actually guilty? Would you say the case is different since other governments can be known to be more prone to corruption than our own?