I do have some sympathy for the landowner, but there is no escaping the fact that the mines are there and that issues relative to this fact will arise.
What has been done is blatantly illegal however, and at this time that is problem no 1 from the landowners point of view.
The mine must be put back to the state that it was before, and if necessary a grill fitted to prevent access.
However, that will then be the start of a tug of war similar to Cwmystwyth between the landowner and people who turn up with hacksaws.
The only solution will then be some kind of dialog between the landowner and interested parties.
Maintaining underground access is desirable also from the landowners point of view as this would enable inspections relative to any instability that could effect the ground above.
The mines will not go away.
My avatar is a poor likeness.