I don't have documentation that this is where the Twain-Cable speaking engagement occurred but the Twain Speaking Engagements site mentions it occurred in the Methodist Church and Janet C. Olson Assistant University Archivist at Northwestern University Library provided me with the address:
The night the opera first came to town
The Grand Theatre was designed by Merle F. Baker and was constructed on the foundation of the Keokuk Opera House (Circa 1880) which burned in 1923. It was patterned after theaters in Chicago and was praised as one of the finest theaters in the country at the time.
http://www.keokukiowatourism.org/theatre.htm
It Happened In Keokuk Friday, December 7, 1923
KEOKUK'S 43 YEAR OLD OPERA HOUSE IS GUTTED IN EARLY MORNING BLAZE.
I have no direct information that the Opera House was the location of the Twain-Cable reading of January 13, 1885, but I found this listing in the Hannibal City Directory 1885-86. Hannibal Opera House Co., cor of 5th and Center, J.B. Price, manager, office at F. & M. bank.
Image: Hannibal, Missouri: Bluff City Memories By Steve Chou
Doerr's Opera House operated out of the second floor of the building on the northwest corner of 6th and Maine Streets.
http://www.idaillinois.org/cdm/ref/collection/qpl/id/1589
The St. Louis Mercantile Library, founded in 1846 in St. Louis, Missouri, was originally established as a subscription library, and is the oldest extant library west of the Mississippi River. Since 1998 the library has been housed at the University of Missouri-St. Louis. It has 600 feet (180 m) of papers, ledgers, and printed materials currently in 26 departmental or other record groups In 1986 the library received a grant from the National Endowment for the Humanities because of the collection's cultural importance.
January 8, 1885
From http://sangamoncountyhistory.org/wp/?p=2456 Chatterton Opera House, Posted on October 30, 2013 by editor
However, on March 17, 1876, Rudolph’s was almost completely destroyed by fire. “There is a story that Mr. Bunn, upon being awakened with the news that the Opera house was burning down, remarked that he couldn’t put it out, and turned over and went to sleep again,” Gib Bunn wrote.
This church has owned and occupied three houses of worship: The original Plymouth Church, northwest corner of Meridian street and Monument Place, now a part of the English Hotel; the second Plymouth Church, on the southeast corner of Meridian and New York streets, on ground now occupied by the Federal Building, and the third, on Central avenue, at Fourteenth street, which was acquired by purchase and remodeled. http://scholarworks.iu.edu/journals/index.php/imh/article/view/5761/5242
The College of Music of Cincinnati was founded in 1878 by George Ward Nichols and funded with a lead donation from Reuben Springer. The famed conductor Theodore Thomas was immediately hired as the director, a fitting choice since Thomas had been informally involved in education all his life by bringing symphonic music to people throughout the United States. Initially classes were held in Dexter Hall, which was adjacent to the newly-constructed Music Hall.