I think the main reason they say one of each endless spell per player is because otherwise it would get super-OP if the more powerful magic users such as Nagash and Morathi were able to summon more than one of the same endless spell in the same turn as they would go around and do all kinds of damage to the opposing army, unless they had a powerful magic user of their own to counter that. Endless spells themselves are a great leap forward that will make magic considerably more powerful in the game, approaching Fantasy-level magic in its ability to change the game - to have multiple copies cast by the same character in the same turn would just be going too far. Also it would be weird if one model of an endless spell came into contact with another model of the same spell belonging to the same player - would the first one destroy the second one due to the carnivorous magics attacking each other?
That still makes it a bad solution. The current solution hurts a lot of "genuine" uses to solve certain specific abusive cases, much like the actual rule of one.
What would be better imho would be to just reign in the power of the spells so that spamming em isn't such a problem. For example, there's no need to have everything do mortal wounds. Hell, arguably it'd open up a bunch of possibilities. For example right now the majority of the damage ones do D3 mortal wounds to whatever they moved across/came near. If one of em instead just did say 2D6 normal wounds it'd actually stand out and serve a different purpose as opposed to having a slightly different minor side-effect.
Secondly, they could add in a mechanic represeting the unstability of the magics. The more endless spells active, the more likely the next one will blow up in the caster's face. Let's say failing the casting roll once there's already one present means your wizard suffers D3 wounds and the casting value goes up by 1 for each one already present. Even Nagash with his 3+ to casting is rapidly going to run into issues then..
Thirdly, add in a mechanic that the spells have a chance to blow up whenever they get near eachother. If they have a 50% chance of blowing up the moment you put them within 9" of eachother it'l already severly limit the amount of spells you can put out as you'l just run out of space.
All of these would add flavor to the magic system, increase diversity and be consistent with the fluff and thus be far better than just "you can only cast 1 of em"
As for Nagash, they could just change Nagash to not be a horrificly overpowered caster. A 3+ bonus to casting when casting values are usually 5-8 alongside the ability to cast 8 spells while most can only cast 1 will Always mean that Nagash is going to be overpowered when the average caster is balanced. The only ways to keep Nagash in check would result either in the average casters being far too weak or by using gimmicky and "lame" rules like the rule of 1 to negate most of what makes him stand out as a powerfull caster. Seriously, with the rule of 1, what's the point of having 8 casting attempts?
Also, stuff's up for pre-ordering.
