Charles:
Let me try to generalize some answers for you.
If you want to use 3DGS as an operator interface for an actual mining operation then I would say no that is not possible. The link from getting data from PLC's, DCS or some other setup is just not there. Normally PLC or DCS data is passed to an operator interface via some communications protocol such as modbus to get the data to an IO server. Once the IO server has the data then OPC / DDE or some other method is used to get the data to the HMI (human machine interface). 3DGS has no such support for DDE or OPC. That is just one of many features that would separate a true HMI like Cimplicity or Wonderware and 3DGS.
If you want to simulate a mining operation then, of course, that is quite possible with 3DGS. You won't be able to directly import .dxf files to use as maps but you will be able to build maps useing WED. You also have the option of useing 3DS Max and an exporter plugin.
Yes, you could build large 3D mines with 3DGS.
The scaleing is arbitrary but typically 1 quant = 1 inch. It is possible to make maps up to 500000 quants square before you run into a Z buffer problem with most video cards. Of course you always have the option of breaking a very large map up into several smaller ones.
I suppose automatic generation of levels is possible if you construct your levels useing .wmb prefab tunnel parts. WDL can certainly handle that since you have the ability to create or remove entities from the world on the fly.
Open GL..I don't think so. Not really a problem since all new versions of Windows support DirectX. You'll be out of luck with Unix/Linux/Mac though.
Entities can be created and destroyed via WDL or the SDK. The SDK will allow for the creation of .dll's but all things are possible with WDL. I think anything else 'external' (outside of WDL or the SDK) would be difficult to get access to. As I pointed out in the first paragraph there really isn't a good way to transfer data to and from 3DGS and other applications. Though with a C++ guru and the SDK there may be something you can work out. I've considered the possibility of useing the SDK to allow for the transfer of data to and from and ODBC datasource but at this moment I don't have the time work on that. Oh, before I forget, you do have the option of reading and writeing to flat ascii files. So if you can get your data into a delimited text file then A5 can easily handle extracting the data from the file.
Streaming video..no, though you can play 'in game' .avi's.
Multiuser mode, yes it is possible to control different entities.
3DGS to Linux...I don't think so.