Re New York Ripper.
Working backwards the character exists, but we don't know much about them. This episode gave us the most information about them (I think). They've stopped killing. They still taunt their victims families.
Dexter pointed out that the detectives theory about the crowbar was close, but that it was probably something longer. Meaning someone who was shorter might need something longer.
Also if Praeter has the New York Ripper's primary killing weapon, but the New York Ripper hasn't been caught, then where is the New York Ripper? They're still taunting, so still alive. So the New York Ripper either IS Praeter, or knows Praeter.
Uma Thurman is a possibility as well. We just don't know enough about her character. She has a mother that she deeply cares for, which doesn't tell us a lot, but it means she probably isn't a sociopath and probably not a serial killer.
It's also very convenient story wise if Praeter is a serial killer. Dexter has no reason to kill him otherwise, he's just a creepy dude. Since Dexter clearly plans on killing Eric Stonestreet (having already killed everyone else), leaving a loose end seems like a mistake.