In GR geometry is equivalent to matter and energy. So dark matter in GR *is* curved spacetime. Except if you're saying it's an intrinsic property of space but then you break that equivalence and do not speak for GR anymore. If you put it down, this ends up being more complicated.
No, you can construct clean solutions to GR that involve all sorts of expansions. The faster than the speed of light thing governs acceleration, not velocity.
