I couldn't tell for sure from the pictures. Is it just warped across the width, along its length, or is it twisted, or a combination of the above? If it is just warped, even in more than one direction, you might be able to salvage it. The key is to have the concave side (the side shaped like a cave) more moist than the convex side (the side that isn't like a cave). If it is twisted, there probably isn't a clear concave side. You could try keeping the side that was away from the plywood more moist, but I would be surprised if it worked.
That is the easy part, unless it is twisted. What I'm not sure about is how you would stabilize it once it is flattened out. It might work to just to put it back in the rack with stickers, but it may just warp back.
Once you've gotten it as flat as you can and stabilized, if it still isn't flat enough, you'll have to do the rest by planing. The narrower/shorter (depending on how it's warped) the pieces, the less material you will have to remove when planing.