During the first part of your post you're assuming you're the one that has to attack from long range, while the opponent is defending. If you go in wanting to attack a blocking opponent, what are you going to do exactly? Again, Kokoro is looking for a single counter hit. If she's at long range, the opponent will have to come to mid range eventually, and your job is to keep them at mid range and not let them get too close. She can also try and force the mid range by slowly going closer and baiting an attack for a counter hit. But, when being on close range with her, getting to mid range is a huge nightmare. She is only good at close range for characters that are slower than her like maybe Bass, and even then, the evasive characters like Bayman can give her issues.
Yeah, you can say that her heichu grab is a huge tool for up close. It is on standing blocking opponents... That's a huge condition, and it requires a lot of risk. It works on holding opponents, but other options do more damage there. It's generally better for it to connect on a blocking or sidestepping opponent. It doesn't work on ducking sidesteps, so that leaves out the likes of Bayman, Christie and so on. To condition the player to block a lot means you first have to put pressure on them with Kokoro, and well, that's hard for reasons already mentioned. Unless of course you stun them first. Getting the first stun is the easiest at mid range.
The opposing player has a lot of options against the heichu grab. They can either duck or use ANY type of attack, and your heichu is nullified. And then, good luck getting out of their attacks. Aside from that, even if it connects, they can hold all of its follow-ups. Considering the majority of the cast is as fast as Kokoro or faster, it's not wise to play her at close range. What can Kokoro do with the likes of Gen Fu up close? Helena? Kasumi? Hitomi? Bayman? Eliot? Christie? Ayane? Jann Lee? Mila? Pai? Tina? Brad? Jacky? Which of them are going to be standing and blocking for your heichu grab to connect? The heichu grab is more reliable when the opponent is already stunned..
You can use heichu at longer ranges also, simply to evade or bait something and hit the opponent. So it's not solely a close range tool. At close range you can easily get hit out of the heichu stance in general. The grab is no exception. Of course, if you're dealing with a turtle, it might be better to stay close, and that's where the heichu grab would shine. But a lot of good players are not turtles anymore like DOA4.