1) Is it possible to block a flail ?
well there is only one weapon which some say that it is the best blocking weapon in the game combined with counter and shield,
and because it is very rare to find a brute with this set, i say we could start searching for this brute and then we can answer this question.
but other than this Option, i have never seen a Flail getting blocked at all. and i had a very long journey with RicoRico and RicaRica fighting with the Flail as an only weapon set.
2) Have you ever seen a sai dealing damages and not disarming ?
well as mantis mentioned the shield will not allways get disarmed.
but as the matter of fact i have seen it a couple of times where the Opponent has a weapon and a shield in his hands and the sai disarmed only the shield but the weapon remained in the opponents hands.
but imo this is only a rare special case of your question and the general answer should be "no"