Whether it's 2050, 2060, or 2070 for going carbon neutral does not actually matter that much. It's one of the main points people are haggling about right now (e.g. in Glasgow at the climate change conference in a few months). Which will feature a lot of #1 enabled by #2. How good or bad our models are right now is going to be of interest to historians only (i.e. #3). We'll dodge extinction but will likely deal with the aftermath of our past actions for centuries regardless. But getting to carbon neutral seems to get us to a stage where we might manage.
The point is that we can call a date for this at all at this point because things are happening that are going to make that a reality. That's because of technical breakthroughs (i.e. scenario #2) that mostly already happened. And more is on the way. So any conservative date policicans call, might end up moving to an earlier date when that happens.
Whether you consider nuclear/fusion part of this progress is a bit of a political topic. My point is that we'll be fine with or without that happening. The "we're doomed unless we bet everything on technology X" scenario is basically just a variant of #4.
Actually, we're just dealing with the logistical aftermath of the results of these technical breakthroughs not being ubiquitous yet. But technically there's no other reason to keep burning stuff (coal, wood, gas, peat, etc.) other than that we are dependent on burning lots of it with our existing and increasingly more expensive to operate infrastructure. It's cheaper to not do that already but it just takes time to get ourselves there. 30-40 years sounds about right for getting that done.
Fixing our infrastructure is much less dependent on new technical breakthroughs (e.g. fusion or nuclear) than it is on simply doing more of the (already profitable) things that are being done anyway. Whether it is building gigafactories for batteries, building off shore wind farms, or deploying solar; it all seems to have decent ROI and popular with investors. Unlocking more capital for this is actually becoming easy and uncontroversial. If you separate the moralism and rhetoric from the economics, even the most conservative climate change deniers would not mind some cheap solar on their roof, lower electricity bills, or a ridiculous electrical ford truck with low cost and operational expenses. Paying less for more is a great way to drive progress and incentivize the masses. That's not to say that further breakthroughs aren't needed. They are, and they will happen as well. And IMHO speed up the agenda here.