1. One option for mitigation, even in the worse-case scenario is planetary albedo modification via nano-engineered particulates injected into the upper atmosphere. Best case scenario is 0.5-1.0 C, which isn't catastrophic. Food production may slack 10% in the middle case, which a shift to more efficient protein could overcome.
2. Yes, there and many sorts of costs, one of which is opportunity costs. Driving the market to a dead-end is silly.
3. Fission is still 30 years of so off. Gen IV Fusion reactors are viable and could be build in scale in the next 10-15. And the high-temp of some of the designs can be used to efficiently synthesize transportation fuels that are easy to use and drop into current infrastructure. Hydrogen is difficult to store and transport reliably in large quantities dues to low density, small molecule size, and metal embrittlement. The best thing going for it are fuel cells, but the cells aren't all that durable and are quite expensive. CNG could be a lower Carbon bridge, but it suffers many of the same infrastructure and density issues.
4. In terms of density and Return on energy invested, they definitely are. They are also not "on demand", which brings a plethora of challenges and added cost to the infrastructure.