Those costs specifically end up being reified in the fee those processors charge to their customers, so we can determine an upper limit to how much is spent on energy in that way.
The same is true of bitcoin transaction fees, no? Miners set the minimum fee that they will accept, and people tack a fee onto a transaction, large enough that a miner will likely process it.
No, because there is a block reward too that subsidizes the miners, so you have to account for that too.
We don't really know the relationship between transaction rate and electrical usage in a mature BTC system, because mining is mainly used to prevent double-spends and the minimum required mining rate to support a given transaction rate is a game-theoretical concern and not a technical one. We can only really observe what has happened so far in the Bitcoin ecosystem.