RE: What are witnesses, what they do and why you need to support them.

Good post. A few corrections/clarifications though:

You will be asked for your active key - not your posting key - to confirm your votes. If you can't find it, go to your wallet, click on 'permissions' and see your active key.

If you're using it, your account password (the one that begins with P5) should be sufficient. No need to look up your active key.
_{By the way, if you're using your account password to log in, however, and your account is valuable to you, consider backing up the original P5 password very well and only logging in with your posting key.}

The 21st goes to a back up witness outside the top 20, based on their rank. Their rank depends on how many mVests they have voted to them.

Rank does not factor below the top 20 witnesses. For all witnesses below the top 20, the accounts are scheduled to produce a block based on the ratio of their delegation votes to the rest of the backup witnesses' delegation votes.

For every block they witness, they receive 1 Steem Power.
...
The top 20 get about 0.1 SP a block.

1 SP per block was the case until the December 6th 2016 hard fork. After that, it got a bit more complicated. Witness block rewards are now a fraction of the inflation of STEEM, which is variable. Currently, top 20 witnesses are rewarded with about 0.19 SP per block generated and backup witnesses are rewarded with 5X that, about 0.95 SP per block generated.