changing from cc/sand to sand only

I started out with sand only. Not knowing of all the complications of cc I then added it to the sand bed.
Now, I would like to convert it back to a sand bed which later I would seed it with live sand.
I have a few ?'s I need answers to:
(1)Do I really need a DSB? Or do I need 3 to 4inches to acheive my goals?
(2)In removing the cc from the sand it was next to imposible to extract all of it. Is it possible to leave some of it in there or will this pose a threat later on down the road? The ratio is 60% to 40%, giving sand the edge. It's about 1.5 to 2 inches of substrate in my 125gal tank.
I would like to hear answers as well as all suggestions. Thanks to all the replies in advance.
 
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