Deliverance of Itza

You are correct. Basic rules always apply, "unless specifically stated otherwise." As you say, Deliverance of Itza is a DD spell, and thus is subject to the general spell limitations and those specific to DD spells - unless specifically stated otherwise. The fact that DoI (or other spells) use a phrase "all every enemy unit within X" is not a specific statement to the contrary. Hence, DoI cannot target units outside the Wizard's forward arc, nor, indeed, units in CC.

No.

Lizardmen AB p. 52: "Lord Kroak is a Level 4 Wizard; however, he knows only one spell — The Deliverance of Itza. If this spell is lost, forgotten, swapped or stolen by any means, Lord Kroak will immediately remember it again and, if applicable, the Wizard that swapped or stole it will immediately forget it."

And one should think that the Forces of Order would never ever stoop so low as to use the Lore of Undeath. 

Yea, another instance of "ghosts of editions past." In 6th & 7th edition, there were no spell types (except magic missiles) and no general targeting restrictions, except one: you could not cast a spell on units in CC. Otherwise, any restriction was "spelled out" in the spell descriptions, which could made them quite cumbersome. The 7th edition version of the "Deliverance of Itza" was not a magic missile, did not contain any targeting restrictions, and included the exemption to target enemy units in CC. Hence, it targeted all enemy units within 360 degrees and in CC.  

That is completely different in 8th. Here, there are general targeting and specific spell type restrictions that apply to all spells, unless stated otherwise. So, a spell description does not need to state in any shape or form that these restrictions apply (that is a priori the case), but it must state if they do not apply.  

In general, “stating otherwise” requires a reference to what is deviating from the normal rules (in casu:. forward arc, CC), which is lacking here.

Indeed, it is easy to prove that “all/every” in such spell descriptions is not “stating otherwise.” Let us assume that the general targeting and spell type restrictions do not exist. In that case, a spell that targets all enemy units that are within 12" would indeed target every unit within 360 degrees and in CC.  

In this scenario, if we wanted to limit this to all enemy units to just the forward arc etc., the spell description would need to read something like:

• The Deliverance of Itza is a spell that targets all enemy units that are within 12", lie within the Wizard's forward arc and are not engaged in close combat. The Wizard does not need line of sight to any of these targets. 

However, the general targeting and spell type restrictions DO exist, and the whole point of their existence is that a spell description does not have to repeat them every time ad nauseam. So, let us start scrapping the unnecessary repeats:

• The Deliverance of Itza is a [direct damage] spell that targets all enemy units that are within 12", lie within the Wizard's forward arc and are not engaged in close combat. The Wizard does not need line of sight to these targets.
  

Lo and behold, that is exactly what is written in the AB. Hence, the spell description does not state otherwise, and normal rules apply.


Note that the word “enemy” is actually superfluous too, since DD spells can only be cast on enemy targets, but DD spell descriptions often retain it, e.g.
"Gehenna's Golden Hounds is a direct damage spell with a range of 12". Choose a single enemy model within range — it suffers D6 Hits"  (BRB p. 494).  

On a more general note: sometimes, GW will make a redundant statement to the effect that normal rules apply. From that, one cannot conclude, however, that the absence of such a statement implies that normal rules do not apply. For instance, the rules of a Special Character carrying a Magic Banner usually specify that he "must be the BSB and cannot be the General." That statement is completely superfluous, because the BRB already specifies that the only character that can take a Magic Standard is the BSB, and that the BSB cannot be the General.  The lack of this statement in the description of a SC with Magic Standard would not change anything, and you could not use its absence to claim that the SC can be the General.  

On the contrary, since it underscores my "basic" point: unless specifically stated otherwise, basic rules apply. Since the rules for Mazamundi (and, indeed, a Slann Mage-Priest BSB) specifically state otherwise, the basic rules do not apply in these cases.