Regarding the Liber Bubonicus discussion, its important to note that the item says exactly: "The bearer can use the Plague Prayers ability from the Plague Priest scroll (page 114). If the unit is a PLAGUE PRIEST, then it can cast Plague Prayers twice in your hero phase."
This means that a Plague Furnace would now have 4 total prayers, its own 2 Noxious Prayers plus the 2 Plague Prayers from the Plague Priest (on foot) warscroll. Since the artifact uses bold "PLAGUE PRIEST", you are to check the keywords of the bearer. In this case, since the Plague Furnace has the PLAGUE PRIEST keyword, the second sentence applies and it gets the ability to use those 2 Plague Prayers twice. Note that the double-cast ONLY applies to the two Plague Prayers, not the Plague Furnace's original two Noxious Prayers.
That means that the Furnace with Liber Bubonicus artifact and the Master of Rot and Ruin command trait would actually get to use three prayers (1 of Noxious Prayers, and 2 of the Plague Prayers) per turn, with rerolls for all of them.
This might get FAQ'd to state that you only get two casts of Plague Prayers if the unit already had access to that ability, or two total Prayers