Leo here is the information in reponse to your E-Mail.

Each section is marked A, B, etc and the rows described below start in the North East corner of that section going north to south and the next row going south to north.
You'll notice the Lutheran church just north of the cemetery and Center street just west of the cemetery.

Section A Row 11 north to south
Hagemann, Louisa November 11, 1851 to May 5, 1899 wife of Carl
Carl A. Hagemann December 7, 1849 to February 12, 1922
Hagemann, Wilhelm C.F. October 19, 1889 to August 11, 1890 son of Carl and Louisa

Section C Row 29 north to south
Hagemann, Ellen 1910 to 1914
Hagemann, Charlie no dates son of Carl & Louisa

Section C Row 30 south to north
Hagemann, Charley A. 1869 to 1943
Hagemann, Wilhelmina May 16, 1845 to August 24, 1904 gatten von Ed Hagemann

Section D Row 27 north to south
Hageman, Henrietta S. 1898 to 1954
Hageman, Edward C. 1893 to 1948

There are also Hagemanns in the Holy Family Cemetery, Lidderdale, Grant Township but I don't have that Cemetery booklet by the Iowa Genealogical society.

As to your question as to why the 1 n was dropped from Haggemann it would be interesting to find out if someone who reads this post could explain why the double n was dropped from many family names.

I know that the double n is a common spelling of German names and have noticed that several family names dropped the one n.

Changes in the spelling of names has been quite common in the past so this doesn't surprise me that your name lost one n.
Going through the Lutheran church records the name Oppermann has now changed to Opperman and the name Hell changed to Hill.

Here are the records from the Zion Lutheran Centennial book.

Baptism:
Hagemann, Dorothea September 24, 1893 Pastor M. Bruggemann
Hagemann, Edward September 24, 1893 Pastor M. Bruggemann
Hagemann, Emma July 15, 1894 Pastor M. Bruggemann
Hagemann, Edward July 5, 1903 Pastor A. Ehlers
Hagemann, Edward December 4, 1904 Pastor J.M. Ansorge
Hagemann, Louise April 30, 1905 Pastor J.M. Ansorge
Hagemann, Hilda December 15, 1907 Pastor J.M. Ansorge
Hagemann, Marvin September 27, 1908 Pastor J.M. Ansorge
Hagemann, Ellen May 22, 1910 Pastor J.M. Ansorge
Hagemann, Viola January 5, 1911 Pastor J.M. Ansorge
Hagemann, Erhard October 24, 1915 Pastor J.M. Ansorge
Hagemann, Adolf March 11, 1917 Pastor J.M. Ansorge
Hagemann, Vertus August 15, 1920 Pastor J.M. Ansorge

Confirmations:
Hagemann, Adolf April 5, 1896 Pastor M. Bruggemann
Hagemann, Carl April 5, 1896 Pastor M. Bruggemann
Hagemann, Minna April 5, 1896 Pastor M. Bruggemann
Hagemann, Carl October 30, 1898 Pastor C.A. Krog
Hagemann, Louise October 30, 1898 Pastor C.A. Krog
Hagemann, Sophie August 11, 1901 Pastor C.A. Krog
Hagemann, John March 29, 1904 Pastor H.W. Saeger
Hagemann, Minna March 29, 1904 Pastor H.W. Saeger
Hagemann, Sophie March 29, 1904 Pastor H.W. Saeger

Marriages:
Hagemann, Herman to Maria Ewert July 11, 1902 Pastor C. Krog
Hagemann, Edward to Henrietta Kai December 24, 1927 Pastor J.M. Ansorge

Burials:
Hagemann, Mrs. Ed August 28, 1904 Pastor H.W. Saeger
Hagemann, Heinrich Edward January 21, 1905 Pastor J.M. Ansorge
Hagemann, Edward September 13, 1922 Pastor J.M. Ansorge

Once I get the church records on the Internet you can double check for errors and omissions.