Demyx's evasion counteracts Axel's speed. And I would assume it would be pretty damn hard to try and run around really fast when you have an army of waterclones all over the place, geysers of water are popping out from beneath your feet, and a barrage of water bullets and jets are flying at you
Axel's Chakrams could in no way block Demyx's sitar. Like you said, they are light, so Axel's Chakrams against Demyx's Sitar would be like someone with a Dagger versus someone with a Claymore.
If the Dagger Weilder attempts to block the Claymore with the blade of his Dagger, is it going to block it just because the 2 weapons are making contact? Of course not, the force of the Claymore is going to knock the Dagger out of the way, most likely breaking the Dagger weilder's arm in the process