now that mana is global, the advantage of having multiple channelers is very slight; more logistical than an increase in total power (so long as the mana gen is balanced of course). so to my mind they should all be natural channelers.
personally i don't like the idea of charging a mana maint for imbuing at all. it has to be dismissable in order to not break the game, but so long as it remains so, it will be open to exploitation. more than anything, it just doesn't feel right lore-wise. the overabundance of mana maint at the moment makes enchanting and summoning in the early game very unwise, as this is the time when you should be stockpiling for the mid game.
i preferred essense as a means of limitting imbuing because it makes imbuing a personal sacrifice for the sovereign rather than a global one; giving the player the choice of being sauron (and hoarding power for himself) or galadriel (sharing power for logistical benefits). i also think this would make the game more balanced. having one stat govern all magic vs 3 stats to govern combat is not a good idea. if intelligence is currently a dump stat, it's only because the damage formulas at the moment don't scale enough, there are no good high level damage spells and the sovereign isn't being limited by intelligence as he should.
if it was me i'd do it like this
INT: required for casting spells based on their level, increases spell power, admin bonus to city income (giving stay at home sovs more options)
ESS: reduces mana costs of spells, determines spell resistance, sacrificed for imbuing, enchanting and creating fertile land.
i'd also add a spell that turns the sovereign into a Lich; preventing them (and their decendents) from having children, making them unkillable permanently without taking their capital and giving them large stat boosts. that would cmplete the game for me.