I'm not really falling into this whole debate too deeply because it seems a bit flawed to me, but even if it was Sheila's palm print (If it even is a palm print) - it proves nothing. What's to say Bamber didn't do it using Sheilas hand after the first shot, to show she moved after that first shot and was capable of firing the second.
But then again - for this to be Sheila's palm print you have to ignore and bend over evidence.