1. Global dimming was observed, but since the introduction of legislation to curb the emissions of particulates, the quantity of these particulates has decreased, so global dimming has actually reversed in the last 10 years.
2. UV doesn't cause heating - it's the infra-red that does. Also, clouds are transparent to UV but reflective to infra-red, so you can still have high UV without an increase in temperature. The amount of each that the sun generates varies over time, as does the amount of other solar radiation.
3. The difference in radioactive decay is fractions of one percent, and may be due to the level of neutrinos being emitted from the sun, of which 60 billion pass through an area the size of an average fingernail every second.
So, while the Sun inevitably has an impact on the climate, the processes that are going on are vastly more complex, but I think it's more likely than not that mankind is having some impact on it.
