I'll put my hand up straight away and say that apart from being moderately well read, I have no authority to comment, it is merely my opinion.
I gather that a lot of the myths of what-is-under-what were dispelled in the 60's. I remember various discussions on paper relating to what was under North Downs/Wheal Busy. I think both died in depth. West Peevor was a different matter though. They quit the prospect, just as they had achieved their goal which defeated them.
I wonder who holds all of this information. It must inform general geological knowledge to a considerable degree. (The stuff of no commercial value). What is also interesting is the seeming lack of 20th cent plans for many operations. The 1820-1900 seems pretty good but after that, there appears to be very little.
It's not on the CCC website anymore, but they had a list of critical shafts with 2 types. Super important ones and less than important ones. If I recall correctly, some of these choices were bizarre, for instance Harvey's shaft at Tresavean. 395F deep (800m odd) and nothing down there good water mine though!!! I imagine the other strategic shafts also have not been built around. It's the ones which are strategic but not on the list which are a concern.
Looking at Kelly Bray, I gather there is a major structure under there and the whole of Kelly Bray mine has been obliterated by a lego-estate. It certainly made me wonder how this would impact the structure and physical operation. Considering the report of the Zn-Cu left under Perranporth, it makes you wonder how much that has been sterilised.
One of the key problems that I can see is that it appears that many council, environmental and NIMBY types cannot see how the economy works and how this stuff and operations are essential for our wellbeing. I fully expect to see the media full of yoghurt weavers and NIMBY's bleating about someone digging a hole in their area.
"Well, you're in Cornwall, init?!?!"