The cases in the first letter only serve to make the point that secular courts are not going to question or get involved with the rulings of an ecclesiastical court. There has never been any case or statute in the U.S. that says that secular courts are going to enforce any duty to follow the internal laws of any religious organization.
The only legally significant part of the 1996 letter is the part where they say their files about members are secret and they aren't turning them over, and that the FOIA doesn't apply to them. Not every word in a letter written by a lawyer has the force of law. Sometimes lawyers try to take the moral high ground, and that's what they are doing when they say you promised to listen to our rules when you got baptized. There is no law that says you have you keep all of your promises, but when a lawyer is dealing with a case of a broken promise, he or she will almost always put in a paragraph about that in addition to any legal basis.
If you really became legally bound to Church governing procedure when you get baptized, they could go and get a court order to compel you to attend a judicial committee if you refused to go. As the maxim goes "for every right there is a remedy." If they have the right to hold people who join to a "legal burden," then what is the legal remedy against those who don't?