The U.S. Congress man arrested was Henry May, Delegate from Baltimore. A couple of day later, one third of the Maryland legislature was arrested. This was to prevent the possibility of a vote of secession. No one actually ever tried to arrest the Chief Justice. He himself thought that he might be arrested after the Merryman decision. There is no solid proof that the Lincoln administration considered arresting Taney. Sure they would have like to, but the only source for the supposed arrest of Taney is from statements made in the 1880s by a friend and bodyguard to Lincoln.
You are aware that what you are talking about occurred in September 1861? Months after the Civil War had begun? And that would mean that the Maryland legislators that you spoke of, who were a minority to begin with, were advocating joining a war ongoing against the federal government? Given all that then what would expect the government to do?
And I had not heard of Henry May before. Thank you for educating me.