I suspect your CYA will stay around 20 as you technically added 17 and presumably started from 0, but it's safe to wait and you are right that it does provide a lot of protection even at 20.

As for the clarifier, it will speed things along by clumping dissolved algae remnants but technically it is unnecessary if you wanted to just use chlorine and be patient since the chlorine will continue to break down the algae. It's up to you. Just don't add too much -- follow the directions on the bottle and you can then vacuum to waste if there's sediment that settles. It usually takes 3-4 days to clear up an algae bloom and sometimes up to a week depending on the size of the bloom.

You might consider cleaning (backwashing) your filter as you've noticed a big rise in pressure.

Richard