they do focus on vaping, that does not mean they are irrelevant to the question of nicotine. from the cdc link:

To this and your subsequent points, these claims are not backed up by sources in the pages you linked to, and as we've seen from the other paper as well, there's good reason to be cautious about assuming their claims are separating the effects of nicotine from the effects of the delivery method, especially given every single source actually cited by the CDC article is about smoking. Neither the Johns Hopkins or Harvard article cites any sources on nicotine alone that I can see.

i disagree that ignoring delivery methods is “meaningless”. form the johns hopkins article:

And, getting hooked on nicotine often leads to using traditional tobacco products down the road.

A claim that is not backed by sources, and has divorced this from delivery method. E.g. how many people starts with gum or a patch and goes on to tobacco? I can certainly see there being some transfer from vaping to tobacco, but that is very different from the blanket claim and illustrates the problem with these sources that fail to disambiguate and extrapolates very wide claim from sources that looks at specific modes of use.

the part you quoted says that nicotine acts as an accelerator for the development of cancers from other sources, including things like car exhaust. these carcinogens are widespread in the modern world, so accelerating the development of cancer associated with them is a bad thing. eg, car exhaust fumes are everywhere.

Yes, inhaling nicotine is bad. That we can agree on, and the source supports the limited claim that if you get nicotine in a way that binds to cites in your lungs, that is bad. The sources do not provide evidence that this risk is present for other modes of use. Maybe it is, but they've not shown that.

i agree, this is bad. the problem you brought up with the “materials and methods” section is also bad. i’m not trying to defend the article holistically, i’m even particularly attached to that source (which is why i included a few different ones).

But that article is the best of the sources you gave. The others cite nothing of relevance to the claim I made that I can see after going through their links.

the article did this by reviewing “90 relevant articles” from PubMed and Medline, then discussing what those articles found

But the problem is that not nearly all of those "90 relevant articles" are relevant to their claim, and so they start off by misrepresenting what they're about to do. They then fail to quantify their claim in any way that supports their conclusion. They back up some specific claims without quantifying them (e.g. I can back up the claim that apples can be lethal, but you'd need vast quantities to get enough cyanide from an apple to harm you, so a claim they can be lethal in isolation is meaningless) or unpacking whether they are risks from nicotine in general, or nicotine via a specific delivery method. This is an ongoing problem with research on this subject.

They have not provided an argument for how any of those "90 relevant articles" supports their conclusion.

i think the second statement was thoroughly debunked by the sources i’ve included: they all say nicotine is highly addictive, and one of them says it’s “as addictive as heroin and cocaine”. i think the sources i’ve shared also discredit the idea that nicotine is “up there with caffeine in terms of safety”. i’m not trying to say nicotine is extremely dangerous, but rather that its danger is underestimated.

The say that, but they don't back it up. Ironically, pointing to heroin is interesting, because the addiction potential of heroin has also been subject to a lot of fearmongering and notoriously exaggerated, and we've known this for nearly half a century -- a seminal study of addiction in Vietnam war vets found the vast majority of those with extensive heroin use in Vietnam just stopped cold turkey when they returned to the US and the vast majority didn't relapse, the opposite of what the authors assumed going into the study. A study that was commissioned as part of Nixons then-newly started politically motivated and racist War of Drugs with the intent of providing evidence of how bad it was.

(see https://www.mayooshin.com/heroin-vietnam-war-veterans-addiction which gives a reasonable account of Robins study, and gives full reference to the paper)

That's also not to say that heroin isn't dangerous or seriously addictive because it is. Nobody should use heroin. But it's also frequently used as a means of exaggerating by implication because peoples idea of the addiction potential of heroin is largely way out of whack with reality and heavily context-dependent. So when someone drags out a heroin comparison without heavy caveats, that's reason to assume there is a good chance they're full of bullshit.

In other words: It's perfectly possible that some ways of taking nicotine can be as addictive as heroin, but that doesn't tell us what most people think it does. E.g. UK hospitals sometimes use heroin (as diamorphine; its generic name) for post-op pain management because it's far better than many alternatives.

The sources you've given do not present any support for claims that nicotine considered separate from delivery methods is particularly risky. They do provide support for claims it's dangerous when smoked, and possibly dangerous when inhaled even via vaping, and the takeaway that you should generally avoid inhaling stuff other than clean air without good reason is good. The other claims about nicotine in general do not appear to be backed up at all.

i’m not trying to say nicotine is extremely dangerous, but rather that its danger is underestimated.

I find the notion that the danger is underestimated hilarious when one of the claims used a comparison with heroin to fearmonger.

Your source, if anything, is evidence to me of the opposite.