I think there are two Nicholas McGree's. One (age 27) arrived in April 1850 on the ship Patrick Henry. The other one (Nicklas McGree, 25) arrived on the ship Universe in Aug 1853, bound for Minnesota with Catherine & Mary McGree (both age 20). The bio for the Minnesota Nicholas says he arrived in 1853. Rosa's Nicholas (age 40) is probably the one who signed in up Brooklyn for the 63rd Infantry Regiment of the Irish Brigade during the Civil War in 1864. They may have been estranged before that, as she and the girls are alone in the 1860 census in Syracuse, or perhaps he had to find work away from Syracuse. There is a Nicholas McGray in Brooklyn in 1860...could be him (Rosa was listed as McGrey). As Rosa repeatedly lists herself in the Detroit directories as "wid of Nicholas", maybe he died in the war.
BTW, does anyone has a source for Rosa being a Murphy?