First of all I very much agree with you, no amount of human made renaturalisation will make the Land the same again. Nontheless I wish to explain the current process that exists in Germany from my first and second Hand experience.

To start, these surface coal mines span a much larger area than the lake that remains at the end, means they are moving through the landscape generally taking multiple years. During active mining plans about the final state of the landscape are already being drawn up and the company responsible for the mine is required to build up finances to fully finish renaturalisation, with is a process that already starts during active mining in areas where the coal has already been taken out of the ground, as they filled up by the earth taken from the active side of the mine. In doing so it is generally planned in a way to have similar soil roughly back in the layer it used to be before, so top soil will be on top again, once the ground is there again, plants are grown. Depending on the area planning there are generally mostly areas for farming and for forests, on farming areas there is a certain set of plants that are meant to be grown to increase the fertility of the ground and I believe in forest areas things may be similar, in any way, those areas are prepared (over multiple years obviously) and are often times already in use/pretty far developed even as the mine is still going.

When it comes to the end of such a mine and the often necessary lake, they are usually flooded through external water input and not the ground water, allowing for a better water quality. This of course doesn't change anything about the lowered ground water level that takes decades or more to recover and the increased evaporation but at least you can swimm in it without the fear of it melting your skin off. Besides that a problem that may occur with landslides on the piled up dirt side of the lake is nowadays generally eliminated with ground ramms to basically make the soil set in a stable position.

As to holes being sold of I cannot say much, I do know tho that they are under "bergaufsicht" so under surveillance by the ministry of the state responsible for mining till renaturalisation is finished and in the case that the original owner cannot finish it for whatever reason or some so far unknown late damages occur the ministry is also responsible.

Over all I wish to say that I am not defending the general practice of digging up mass amounts of earth, destroying existing eco systems, sometimes destroying villages and leaving a lasting impact on the area but I just wanted to at least mention that at least a whole lot is being done to try and mitigate the effects it has...in Germany...currently...