I've found it often enough to not notice much. The best birth date I can find on her is Sept 2 1860. In the 1880 census she is listed as the oldest child at age 19 (dates fit)named Mary F. Hoover.

More on Francis H Horn:
Cemetery Records for Cedar County, Missouri Volume II
Name: Francis H. Horn
Birth Date: 05 Jan 1845
Death Date: 12 Jan 1920
Cemetery: Hackelman Cemetery
Description: Located about 7 miles north of Stockton on Highway 39 and one half mile west.

So he didn't die in the Civil War for sure. He was born and lived most his life in Cedar and Stone Counties. So maybe he isn't the right first husband.

He was son of Dr. Samual Horn, From Goodspeeds Hisotry of Cedar County:Pgs. 742, 743

Dr. Samuel W. Horn, the oldest physician and surgeon of Cedar County, Mo., was born in the State of Georgia, near Augusta, in 1811. His father, Dr. John Horn, was a Virginian, born in 1778, and was a medical graduate and soldier in the War of 1812. At Norfolk, in 1814, he was thrown from his horse and died the following day. His wife, whose maiden name was Charity Brown, was born in Georgia, and died in 1836, at the age of forty-five years, in Smith County, Tenn. Her two children are living: Dr. Samuel W. and Miranda, wife of Allan Horn, of Greene County, Ark. The former was three years of age when his father died, but he continued to make his home with his mother, and went with her and his step-father, Henry McMullen, to Tennessee. He was reared to manhood on a farm in Smith County, and in 1829 began teaching school, continuing this occupation for four terms in Mount Holly Academy, and at the same time pursued his medical studies, using his father’s books, which his mother had preserved. He also studied under the direction of Drs. Ben R. Owen and John Daugherty, and in 1833 entered the Medical College of Louisville, Ky., from which institution he graduated in May, 1834. He began practicing at Lancaster, and in May, 1835, married Miss Margaret Tyree, who was born in Smith County, Tenn., in 1822. Of their ten children seven reached maturity: Mary, who died in 1863, at the age of twenty-five years, was the wife of William Cawthorn; William J., Francis H.; Samuel W., Jr., who died in 1870, aged twenty-two years; Martha L. J., wife of Joseph C. Ledbetter; Alexander M.; and Miranda C., wife of William C. Preston. In 1843, Dr. Horn left Tennessee and went to Mississippi, and in February, 1844, landed at what is now Cedar County, Mo., and located on the farm where Thomas Hackleman now resides, where he entered upon the practice of his profession, there being only three other doctors besides himself in the county, and he is the only one now living. He devoted his time to his profession until June, 1882, and since that time has lived a quiet, retired life. He had only been in the county a short time when his worth and merit as a physician became known, and for over twenty years he had the most extensive practice of any doctor in the county, and very often was called a distance of forty miles from his home to attend the sick. Many a time he has been on horseback a week at a time, with but little rest, and often fell asleep on his horse, and one time was knocked from is saddle by the branch of a tree. Another time his horse stopped, and he slept for several hours before waking. During all his years of residence in Cedar County, his good name has remained untarnished. He is known the county over as “Uncle Sammy,” and is reverenced by all who know him. Previous to the late war he was a Whig in politics, and voted for Henry Clay for the presidency in 1832; since that time he has been a Democrat, but during the War was a strong Union man. In 1850 he took the first census in Cedar County, and was one of the original stockholders of the Bank of Exchange of Stockton. He has been a member of the Masonic Order for forty-four years. In 1885 he lost his estimable wife, and since that time his youngest son has been living with him. He has given each of his children $2,200, and has always been very liberal in contributing to worthy enterprises. On coming to the county he had not a relation, but now, besides his own children, he has thirty-nine grandchildren and sixteen great-grandchildren. He has been very successful financially, and is one of the well-to-do men of the county.