Isn’t saying kaddish for a niftar a bit similar to doing teshuva for him?
No.
First of all, as I explained above, you cannot do teshuva for someone else. It’s a personal process. You can’t do teshuva for someone else no more than you can eat for someone else.
You can argue that saying kaddish may be a kapparah for the sins of the deceased (see my post above about the difference between teshuva and kapparah). It may also just be something that elevates the neshama. But it’s not teshuva.
Whatever the kaddish is doing, couldn’t one do/achieve something similar for a living person?
Perhaps. You may or may not be able to achieve a kapparah for someone else. But not teshuva.
The Wolf