How much does Diffusal Blade slow enemies?
According to this website: http://dota2.gamepedia.com/Diffusal_Blade
The movespeed slows 100%, and then the slow decreases in 0.8 second intervals, so it slows for 100%/80%/60%/40%/20%, going one step down every 0.8 seconds.
This page also has some info on the buff: PURGE http://dota2.gamepedia.com/Purge
As for the slow stack, I believe it does but I don't think you can slow a hero beyond 100%. That means, once the slow goes 80% or lower then you can see the effects of the other stacked slow items.