That's indeed the reason. Simple fix is to set the rendertarget usage also for ddraw offscreen surfaces.Not sure if there are any possible sideeffects of this, but until now i haven't seen any.
That looks a bit more complex. I changed the way ddraw and WineD3D interact some time ago, but I didn't update the palette code for that change. I think I have to re-write the palette setting code, it's pretty crappy ATM.