Regarding Kokoro:
If you're caught by Kokoro's 6P, either by blocking it or getting hit by it, you cannot step the second hit. 6PP as a block string is fairly good, despite certain short-comings (duckable on the second hit), neither JAK nor Mila's SS 6T can effectively duck the second hit if they sidestep. If Kokoro is delaying? Possible, but as a non-delayed string, 6PP is a good check to keep players from side-stepping Kokoro. And this is why I don't feel Kokoro really has a issue with step.
If players are stepping her 6P, then yes, you can punish before 6PP comes out to punish the step, though it would depend on the move (super fast strings or TC strings would be enough. Hitomi's SS P, Jacky's SS P_K, JAK, etc). As a string that checks step, 6PP is pretty decent and its arguably one of Kokoro's few good strings to really abuse.
6PP let's Kokoro actually play a mix-up game that she doesn't really have. It gives her a nice frame advantage, checks step and has variant mix ups in 6P6P and 6PPP with the last hit being infinitely delayable. 6PPP also works well in wall combos. And it works well in punishing offensive play at negative frames (though Ayane's, Kasumi's and Lei-Fang's are better). A successful 6PP let's Kokoro get away with a lot, but its not the end-all and be-all of Kokoro's moves. Its an enabler to let her do what she wants, especially with her limited tools, but a good player will know this about Kokoro, and every character and good player will have ways of shutting it down This is why 6PPP exists, and probably why 6PPP got a buff for the upcoming patch.
Full stop. Kokoro's only mix up is her PK or 3K string variant. Its her only string that let's her strike at all four heights (high, mid punch, mid kick, low) and has at least two guaranteed options. All but one of them is linear, but the tracking high is difficult to recover from if you're hit by it (and you will be if you attempt to step after a blocked K). Perhaps one of the reasons why players find this string so difficult to deal with is because of that tracking K and why no one effectively steps the PK string. It kind of reminds me of Natsu's 2A+B low. Its a super slow low, but the telegraph is barely readable and ties into her unblockable. Even in the meta-game and knowing the faults behind the move, its still tricky to deal with effectively. I feel like this is perhaps one of the reasons why Kokoro can get away with a PK2P (and variants) and see that low land all the time despite the telegraph and slow speed, because the string is so varied that players aren't quite sure where the last hit will come from, and because 6PP, sabakis and opening variants (PK, 3K, 1KK, BT 4PK, etc) make it scary. A bound launcher on one finish, a positive low on another, a safe BT setup, and tracking on the last hit seem to make a lot of players question whether to take the risk to shut down the string or step it. In this sense, this is Kokoro's only string mix-up, but its an incredibly good one. Especially when paired with 6P, 7P, and throws.
Kokoro's 66P+K is not a very good string to advance with, if the first hit (mid) is blocked, its completely possible to hold the last hit (high) and punish it completely. The Heichu follow ups (throw or strike) don't guarantee protection from punishment either and the move, overall, is pretty slow. There are far better mid options. 66P+K is best used as combo filler and the upcoming tweak is intended to make this move used more for that than attempts to guarantee damage from a sit-down stun or an advancing mid.
