I'm not anyone exceptional, but will try to answer a few of your questions.
Not great priority -- Nu's standard antiair stuff beats it clean. Unsure about other chars, but it's not nearly as good as it looks. In a blockstring I don't think it's quite fast enough to be effective, but I could be wrong on this point.
2D is unsafe at the end of a blockstring, as you'll be close enough to be punished. j. D or tsubame will both put the staff on the ground as if you did 2D (meaning the D return will be low).
You should be outranging 2A pokes as Litchi IMO: use 6A , 2B, or 3C depending on the distance. At extremely close range, Litchi's moves (except tsubame) won't come out fast enough to beat jabbing -- unless your own A poke is faster. Chun outside of a combo is dangerous, incidentally -- you'll eat combo if they block it.