The above answers contain good arguments about the drawback of PoW
scalability: PoW schemes require propagating new candidate blocks to all miners around the world, and occasionally miners will split and do work on a chain that ends up being orphaned / not the longest. This happens for brief moments all the time, and in Ethereum these "almost blocks" help ...
EV means expected value. It is the average you will receive if you repeat the experiment a large number of times.
p=0.9 means if you choose that branch you have 90% probability of it being the correct one.
The locking happens when you "vote".
You can vote both alternatives. They analyze that case and propose possible solutions to prevent that behavior.