I haven't sent a letter to a congressman myself, but I've seen people reporting success from time to time. It would not hurt your case for sure. The biggest slow down for N-400 is usually pending I-751. Now that you have it approved, I think your case may move faster in January-Fabruary next year. I wouldn't believe the estimate onlines, they're never accurate. As to renewing Ukrainian passport, I heard they can get extended too? Though it would be somewhat tricky travelling with such extended passport in my opinion. Good luck with your case and I wish you a speedy approval!