I think a solution would be to eliminate the PoW in the second round and simply make it a random selection (somehow). That would cut the expected block reward by a factor of N2, which would cut the power usage in the first round by that same factor.
Not sure on that. But why not just have a single round 1 and just select the winner at random without eliminating anyone? Why wouldn't that work?
The PoW job is also to maintain the Block Time at 10min. We can achieve that by just making everyone wait for that 10min minus delay/network broadcasting time etc.
I think you need to have some skin in the game if you want to have long term serious players in the network. PoW is achieving this skin in the game by asking for CPU power and PoS is achieving the same by asking for stakes.