1. There is no relation between PoW energy consumption and # of transactions so talking about energy used per transaction is nonsensical.
There will be in a few decades when the block subsidy dwindles into insignificance and the network security is mostly funded by transaction fees.
That's just simply not true.
The lack of relation between PoW energy consumption and number of transactions has nothing to do with the fact that there is a block reward. The two systems are entirely unrelated. When miners get mostly, or even only, tx fees the mining energy consumption and number of transactions will still not be related at all. Number of transactions is decided by block time and block size limit, which have nothing to do with amount of energy used by miners.
Think about this: Would number of transactions change if a year from now mining was 1/10th what it is today, or 10x what it is today? Nope. It would have zero effect on number of transactions processed. Transactions will simply continue to stay at the limit of what Bitcoin can handle no matter how much or how little energy is used.