Question about energy usage: roughly how much does it increase as a function of transaction volume? If 10x the number of people start getting involved and transaction volume increases 10x as a result, will the network use 10x the energy?
It has no relation to transaction volume. It is related to the price of BTC and the block reward every 10 minutes. The higher the price, the more miners compete to find a block. Transaction volume can never increase because they have capped the block size at 1mb.
But only if it's profitable to miners coming in. Ultimately mining pressures miners into finding cheap sources of electricity. Yes, many just take advantage of subsidies (governmental or environmental) to lower their costs, but as mining difficulty and competition increases inefficient are kicked out.