The only William Rowland of Boone county in the right age group is this one in 1850 census

Name: William H Rowland
Gender: Male
Age: 8
Birth Year: abt 1842
Birthplace: Missouri
Home in 1850: District 8, Boone, Missouri, USA
Line Number: 3
Dwelling Number: 61
Family Number: 61
Household Members:
Name Age
John C Rowland 35
Ellin C Rowland 30
Nancy E Rowland 11
Mary A Rowland 10
William H Rowland 8
John H Rowland 6
Sarah F Rowland 4
Martin E Rowland 2

In 1860
Name: Wm H Rowland
Age: 18
Birth Year: abt 1842
Gender: Male
Birth Place: Missouri
Home in 1860: Rocky Fork, Boone, Missouri
Post Office: Columbia
Dwelling Number: 189
Family Number: 185
Attended School: Yes
Household Members:
Name Age
Wm H Rowland 18
John Rowland 16
Frances Rowland 13
Martha Rowland 12
Julia Rowland 10
Lara Rowland 6
David Rowland 3
Not named Rowland 1/12

Note he is in the Rocky Fork Twsp which was a hot spot for Purcell.

Then there is this entry in History of Boone Co.
After the second Centralia massacre, September 27, 1864, William Rowland was in Centralia in a wagon with some ladies, when the guerrillas came into town on the heels of the Federals. Tom Little, one of Anderson's guerrillas, rode to the wagon and struck at Mrs. Redman, one of the ladies, with his revolver. Mr. Rowland caught the weapon by the barrel and held on with such a grip that he was almost dragged from the wagon. Little then killed him. Mr. Rowland was a well-respected young man, who had never been a soldier, but was regarded as a Southern sympathizer. William was an assistant railroad agent in Centralia.

From: History Of Boone County, Missouri

The matching find a grave record is here
https://www.findagrave.com/memorial/112740340/william-rowland

I think William H Rowland may be a 2nd cousin to Samuel and Marion Rowland. I find it a bit strange that Switzler in the history would refer to the same guy in two significantly different fashions. I think I lean towards Marion being the escapee.