The temporary transition to coal was studied long time ago and identified as the best solution even though it would increase CO2 for a small time. But in compensation it would allow to transition faster to green energy thanks to savings done on nuclear, hence overall reducing faster CO2 emission.
When and by whom? The "phase-out" in 2002 was enacted by a government led by a person who later turned out to be a paid Russian agent. How did that work out? Germany did almost nothing for 10-15 while it increased it's dependency on fossil fuel imports from Russia...
The 1.5 step that climate activists ignored (because why not? Humans are cancers. Industrialization is a mistake) is the crashing of economy. Germany is will paying for that mistake and will never recover.