While I'm still learning the character myself, I'll say what I know.
1) When it comes to using Neutral D's, I alternate between 5D and j.6D when my opponent is grounded. J.6D has impressive horizontal range from the air, which makes it a good tool. If they managed to get caught by it, you can go into a small j.D combo for some damage, then proceed to establish oki. When you use 214D, you're more than likely going to want to catch your opponent attempting to jump out of it with 2D or 6D. If they chose to remain still and block 214D, or if they blocked your 2D/6D, you can go into 5D > 236D to lock them down.
2) This is a blockstring I use as well, sometimes I'll do 5C > 6C > 4D > 5D to trip them up if they seem to be blocking the other blockstring you mentioned well enough. The thing about this blockstring is that you're free to go into 214D, 236D, 236A. You can use the moments after your last D to either attempt to lock them down with 214D/236D, or continue pressure with 236A. If you feel as if they'll mash after you teleport, then try to bait it with 22D. It's also important to integrate 6B into your mix-up, as well as TK j.214D. If you're familiar with TRM setups, those will no doubt help you too.
3) Blocking is something that varies by character, and a thing that will no doubt have to be learned. All I can really say is, try not to get pushed to corner, and don't forget about Counter Assault and our reversal DD.
I hope I was able to help in any way.