Slashers are entities that enforce the two above rules. If a slasher determines that you're node is down, or that you're committing an attestation violation (i.e. that you're signing more than one attestation in a given epoch mainly). The slasher actually does not get rewarded here. The block proposer who takes the broadcasted slashing and and adds a proof of it to their block proposition, and get what's called a whistleblower reward. Slashing is not meant to profitable, and the whistleblower reward is quite small. We don't need a million slashers, in fact, we could operate with just one... expect the Eth Foundation to run them, among other large players who can spare the resources.
2. Owning 2/3 of nodes economically is kind of silly. You essentially just 51% the network, and everyone else leaves. You're then stuck with a worthless personal currency. Have fun!
3. More than one validator means more validator rewards, and a bit more "power" in that you're a larger component of the general validation network.