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Is there a way to use yield blocks to implement an IEnumerator<T> which can go backward (MoveLast()) as well as forward?
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No, the state machine generated by the C# compiler is strictly forward.
It doesn't even make sense to go backwards in many cases. Imagine an iterator reading from a network stream - to go backwards, it would have to remember everything that it had ever read, because it couldn't rewind time and ask the network for the data again.
(Ditto anything that generated data in some lossy way. Imagine an iterator which returned a new board for Conway's Life on each iteration - there are multiple boards which could all have been the previous one, so to go backwards you again have to remember what you've already returned.)