There is NO Roger Taney, R. Taney, R. B. Taney in the 1850 Baltimore CITY census (or in Baltimore COUNTY for that matter). There are NO Taneys in the Baltimore City 1860 census.
There is ONE Merryman in the 1850 Baltimore CITY census, and it's not John. There are only 3 Merrymans in the 1860 Baltimore CITY census, none of them John. There are numerous Merrymans in the Baltimore COUNTY censuses.
John Merryman would be less than HALF Taney's age in 1861.
It is well established Taney maintained a residence in Baltimore for more than 30 years.