Can Benediction of the Blessed Sacrament be used to conclude the Divine Mercy Novena?

NO. Benediction is part of the Order for Solemn Exposition of the Holy Eucharist. This order of prayer is part of the Church's official liturgy. It is no longer permitted for benediction to be added to end any other service or devotion. Other devotions, [Stations of the Cross, novenas, etc.] although good and commendable, take attention away to a different object and should therefore be assigned to another time, either before or after exposition and benediction of the Blessed Sacrament. The USCCB has a more complete explanation on their website.