Well there's really no safe level of anything. Literally everything you do increases your odds of dying. It's all about proportion. Breathing doesn't increase your odds of dying very much.

The conversation around nicotine is severely lacking in comparisons of scale. It's all like "IF YOU SMOKE LITERALLY ONE CIGARETTE YOU WILL DIE OF HORRIBLE CANCER" which is a ridiculous thing to be putting out there. I get the intent, it's because cigarettes are addictive and smoking one usually leads to smoking more, but it really makes it difficult to get a sense of how harmful cigarettes actually are at lower dosages.

That first study you linked was a good one in this regard, but the summary still couldn't help but tend towards "you smoke at all, you will die". A graph would be helpful.

Actually, I'll just make one.