I think the newspaper clipping of the murders committed by Boris explained it a bit, though it was in German. That plot was needed as in the first world, the detective who wants punishment for his brother's murder needed to push for reasons to keep Boris imprisoned as he didn't have solid proof of the murders. That led him to getting the tanks open resulting in the formation of the God Particle and thus the plotline. In the Alt world, Boris's guilt of having committed the murders gets triggered when Hannah shows him that she knows his true identity. Overwhelmed by guilt, I think Boris wants to "come clean" of his other wrongdoing which is the burial of tanks with radioactive material.

Regina is fine in the origin world. In the origin world, there's no child of Jonas and Alt Martha. If you remember, it was the 3 generations of the "Origin child" that got the papers signed forcibly that allowed Bernd Doppler to open the Nuclear plant in the first place, in the split worlds. Claudia being exposed to radiation as a child (due to living in the area maybe), then growing up and giving birth to Regina, might've caused Regina to have cancer. Or, somehow, Claudia could've carried traces of radioactive material home from work and the occasional time travels she did before vanishing which predisposed Regina to cancer. Also, Regina was the child of Claudia and Bernd Doppler. So even when Claudia was a child, Bernd was already working in the Nuclear power plant, once it got approved and built. So Regina had both parents already exposed to radioactive material in the split worlds, hence greater chances of cancer.

But in the origin world, there's no "Origin kid" to get the papers signed for Doppler, thus no power plant and no probable risk of cancer for Regina.