I have all my information from my grandfather who was one of nicest and kindest men I have ever known. He died in 1952at the age of 72.
He was born in Silesa (Schlesien) in Eastern Germany, now Poland. The town was Heinrichswalde and was located near the town of Graz. It was called the Grazer Eck, (Grazer Corner). This is not too far from the former German City of Breslau, (now Wroclaw). His father was a peasant or farmer, and they had something like a dozen children. They were not well off and as only one son could inherit the farm, all the rest left and went as far as he knew also to America. He only ever had contact with one brother named Paul and with one Sister, who had become a nun.
Something you have missed is that there are two definite different spellings of the name Nentwich. One version is Nentwich and the other is Nentwig. In German both versions are pronounced identically and you cannot hear the difference. The pronunciation is Nentvish, with the sh pronounced like the typical German ch. In addition to this the old German script, when not written clearly can make the ch and ig look very close.
As well as all this, when a child was born and the father or mother went to the church to have the birth recorded by the priest, simply told them their name. It was then up to the priest to get the spelling right.
To complicate matters, his parents were most likely illiterate and could not assist even if they wanted to. It is for this reason that half the children spelled their names Nentwich and the other half Nentwig. I have two documents for Franz Nentwich, one spelled ch and one ig. Quite a few of the family went to Breslau as their first stop. He told me that there was a Nentwich Street or Square dedicated to some General of that name.
I have tried to get some records from the Heinrichswalde Register but it appears that is was destroyed when the Russians captured the town. The Family was catholic and only the evangelic register survived. All records were kept only by the churches. If you find a map, look for Wroclaw (Breslau). Heinrichswalde is due south or so. I cannot find it on any map as all the names have changed to Polish names. Good luck.