Is it a mortal sin if, being invested in the Brown Scapular as a layman, one does not pray the Rosary daily?
Of course not. There is no obligation at all–the scapular is solely for our benefit. You can’t be made worse off for wearing the scapular You may not receive all the extra graces, but you definitely won’t lose grace.
Absolutely not and if I remember St. Montefort correctly, not saying it would not even be a venial sin. The Rosary along with all Marian devotions are completely free meritorious acts.