The Germans may have erred in retiring workable nuclear plants before the end of their lifespan. If the plants are still usable, then operate them.
However...
Moving to coal is a good idea. Doesn't Germany have decent coal reserves?
If so, then USE THEM.
They have the technology to burn it as cleanly as possible.
Might as well use the resources they have.
They might even consider coal gasification plants -- isn't that a proven technology, as well...?
"Fishrrman's law of energy resources":
Any and all energy resources will eventually be used when needed.
The only variable is how badly the need becomes to extract and use them.