Retiling bathroom questions

I'm a newbie at this stuff and am starting to think I bit off more then I can chew so any help would be appreciated.
Retiling my bathroom....cut out the existing tiles that were on the wall of our soaking tub (about 12" high). Now I replaced the cut out sheet rock with cement board and want to retile up to about 18". My problem is the cement board isn't flush with the existing sheet rock...the cement board is about 1/8" too thin in some places up to 1/4" in others. What's the easiest way to correct this? Do I try to use shims behind the cement board or use wall compound to level it out?
Also, our bathroom currently has small white tiles on the floor. When I break them out, underneath is cementboard but also a ton of thinset. I can't for the life of me scrape the thinset off. What are my options here? Do I have to replace the entire floor cement board or can I use leveling compound? Or sand it off?
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For both issues, leveling compound is great. But, you must work fast.
Of course, if you don't tile over a flat surface, the tile will definitely crack.
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