Would it be possible to take the extra heat from mining and the co2 to make a really nice greenhouse that can use the co2, seems like plenty of space to make it happen in montana.
Sure, but these people care about extreme profitability at whatever external expense. Something like that, while nice is unlikely to generate more revenue than spinning up another plant and pumping the CO2 into the atmosphere
At this point we are like cockroaches. We ain't going anyhwere anytime soon. Parts of our economy might and overall we might regress somewhat, but humans won't die off.
Most Greenhouses supplement CO2 to increase growth in plants since there's no natural O2 -> CO2 back-cycle in the greenhouse. So yes, Greenhouses use CO2.