They can't change a character out of it's slot. Imagine if they removed Roy's kill power! That would completely change what he is in this game. Imagine if they buffed Mewtwo's throws, that would completely change him from the defensive punisher he is now to a powerhouse that is high tier, and great as it would be, he would be OP and he wouldn't need to rely on punishes anymore.
How would making his punish game decent make him less of a punisher, exactly? I'd like to hear more if I'm missing the point. A combo throw is a good punish you earn from a punish. You would still bait and punish stuff, just with the reward Mewtwo deserves and doesn't get compared to other characters. If his role is to bait and punish well, and he can't bait and punish as well as more well-rounded or stronger characters, does his role and playstyle stand out? I think there can definitely be improvements to Mewtwo.
High tier isn't op, it's an ideal balance imo. As a high, you're not limiting the rest of the cast as a definitive top character, and you're not limited by what mid and lower suffers.
More power is ideal for him, since he seems to be designed to be a true glass cannon. He should have the power to compensate and make up for it. If he eats punishes and dies early, but he lacks in punish game and killpower, does he fulfill his role or act as a threat? Being a threat does fulfill what he is, a legendary Pokemon.
With combo throws, buffing a throw simply doesn't mean a few on-paper ideas will create some ridiculous string or kill confirm since there will be careful balance around it. For example, Ike got much better down and up throws recently, but did that change his playstyle or give him anything truly unbalanced? I also don't believe it'd change Mewtwo's playstyle at its core. I love Mewtwo but there's definitely many flaws about the character that need to be fixed or made more user-friendly, in ways that won't change how he plays. It'll just make him succeed in ways that he should.
The combos you listed are alright but not as rewarding as Mewtwo play should be. The confirms aren't ideal, and so much is far from guaranteed. If someone can prevent all of your good punishes by holding shield, knowing that you'll get so little from a grab, then it really hurts Mewtwo. He has bread and butter, but he's carrying around a few buttered pieces of lightly-toasted toast with a toaster he has to find outlets for while everyone else is throwing around loaves of bread and sticks of butter with portable toasters.
When I'm fighting a Mewtwo that outplays me and is clearly more skilled while doing all the Mewtwo stuff right and showing me new things, but I consistently get better punishers and earlier kills to win while they're behind quite a bit, I can't help but feel bad. I don't see much underrated potential in Mewtwo like I do with Toon Link, Meta Knight, Robin, Ike, Olimar, Falco, Wario, and Peach. You're free to prove me wrong if I am via results. I do see underrated potential if he receives proper additions, and want to look into him if that's the case. I feel like my colon's about to pop whenever I try to play him with friends when his shortcomings rise a lot, even when I do Mewtwo stuff right or do cool things. I can outplay someone well and lose, and I'm not exactly neglecting his better punishes and options.
If you're curious on ideas I have in mind for Mewtwo improvements, let me know.