You can find plenty of past debates and arguments on this very subject here on the Force Unleashed forums. All of your points have been discussed before, except your #2. My short answer for that would be: Yeah, memory isn't stored in DNA, but this is the force we're talking about and this IS Starwars. So the idea here is that if Marek is indeed a clone, then his spirit from his original self must have transfered to his clone body or something, like in Dark Empire when the Emperor was resurrected and his force spirit took over his new young clone body. The only problem with that theory is that ALL of the clones of Marek had the memories of the original according to Vader, and they all failed except the Starkiller we control. I would like for there to be a good explanation that this is the original Starkiller or a VERY GOOD explanation saying that he's a clone.
I just really hope that they explain themselves good for TFU3, if it does come out after all.
"All too easy."