2) This doesn't take into account anthropic effects - we have to have survived to get to where we are now. Looking at the past and saying "hey, we survived that!" doesn't mean that the probabilities were that high.
3) The idea is sufficiently well developed now that it's origins are irrelevant (there are few hippies pushing the idea currently).
4) They are computer models, based on extensive knowledge of atmospheric condition and science. Are they definitely reliable? No. Are they more likely right than wrong? Probably - it's not like the underlying science is particularly difficult.
At what probability of the models being wrong would you say that we can ignore the threat? Are you convinced that the models have at least that probability of being wrong? And if so, based on what - it's not like there's a default position "nuclear winter can't happen" that has a huge prior in its favour, that the models then have to overcome.
This is a topic I frequently see misunderstood, and as a programmer who has built simple physics simulations I have some expertise on the topic, so perhaps I should elaborate.
If you have a simple, linear system involving math that isn't too CPU-intensive you can build an accurate computer simulation of it with a relatively modest amount of testing. Your initial attempt will be wrong due to simple bugs, which you can probably detect just by comparing simulation data with a modest set of real examples.
But if you have a complex, non-linear system, or just one...
The FHI's mini advent calendar: counting down through the big five existential risks. The first one is an old favourite, forgotten but not gone: nuclear war.
Nuclear War
Current understanding: medium-high
Most worrying aspect: the missiles and bombs are already out there
It was a great fear during the fifties and sixties; but the weapons that could destroy our species lie dormant, not destroyed.
But nuclear weapons still remain the easiest method for our species to destroy itself. Recent modelling have confirmed the old idea of nuclear winter: soot rising from burning human cities destroyed by nuclear weapons could envelop the world in a dark cloud, disrupting agriculture and the food supplies, and causing mass starvation and death far beyond the areas directly hit. And a creeping proliferation has spread these weapons to smaller states in unstable areas of the world, increasing the probability that nuclear weapons could get used, leading to potential escalation. The risks are not new, and several times (the Cuban missile crisis, the Petrov incident) our species has been saved from annihilation by the slimmest of margins. And yet the risk seems to have slipped off the radar for many governments: emergency food and fuel reserves are diminishing, and we have few “refuges” designed to ensure that the human species could endure a major nuclear conflict.