The simplest way to protect against the time warp exploit, in all its forms, is with a simple rule about blocks whose timestamp is before the previous block.

Just make an adjustment to the difficulty baseline so they don't get the benefit of difficulty adjustments made after their own timestamp. 

In other words,  if a new block is timestamped prior to the previous four, you proceed as if the base difficulty were the same as it was prior to those four blocks.

poof, every time warp exploit in the world becomes impossible.