@shylarahOkay, let's try to unpack this.
We didn't really talk about etheric magic and children, so I'll use Ziotea as an example. When she was a child, she could use her unique ability, but in no way was it as powerful or focused as it is now after twelve years of etheric training. She could knock over doors and merchant stalls, but she wouldn't have been able to destroy a tank. She would also most likely have been pretty winded every time she used her power as a kid. So yes, children do use magic spells and suffer an ether debt as a result, but the magic children use is several magnitudes weaker than what they eventually learn as Inquisitors, and so is the cost. Remember how insane some of our abilities are in comparison to what they looked like when we were children. Ragnar for instance could only summon a solitary aegis when he was a kid, but can now summon an aegis large enough to cover a hundred people and keep it maintained for as long as he needs to. Their magic has grown by leaps and bounds, and so has the ether cost of these abilities. The Red Seminary wants its Inquisitors to be the terrors of the world, and since they have ample ethereal stock to fuel this magic, they don't put a limit on how powerful they allow a pupil's magic to become. Really, I think you're over-complicating this a bit. Think of your spells as like, your "special move" in anime or what have you. It's something you don't rely on all the time, but can bust out when you really need to put a hurting on the enemy.
By the way, the whole blowing up tanks thing... I only suggested it because honestly, her current magic abilities seemed kind of weak in comparison to everyone else's. Stina can lift a thousand pounds for christ's sake! Tatiana can summon demons! Remember the word I used to first describe spells, "miracles"? I meant that! You can go a bit crazy with your abilities, just as long as they're limited in some way and have room for growth, and all of you have done a great job of keeping your characters balanced, it's just that offensively I would've liked her to be a bit more dangerous. I'll tell you right now, the other Inquisitors out in the world can do some crazy shit. Still, if you'd rather keep her abilities as you wrote them, that's fine by me.
There's one concept that I talked about a little bit in the OP, and those are general etheric abilities that even soldiers can use. Here, I'll repost that bit here:
"Besides these 'ascended' ether skills, most common soldiers and all priests are taught the ever-important basic skill of 'ether manipulation'. Every human being has the capacity to learn how to control their own ether, as this allows them to augment their own natural abilities by a set amount. Common infantry are all taught to manipulate their ether to augment their strength and reaction time to a level above ordinary civilians. The level of skill in regards to ether manipulation varies by individual, as well as the pooled amount of ether within human beings. It is known that children with a higher pools of ether within them are sought by the church to become priests and inquisitors."
So, the amount of ether you need to do this is minuscule, since even soldiers can do it. A soldier for instance can focus his ether to be able to run a lot faster, or to help steady his aim when it's freezing outside. This will cost a bit of ether, enough that they'll be a bit winded and sluggish afterward, but it wouldn't require a vial of Omestrian ether, which is only used for really powerful magic.
In addition to these common ether skills, there are some that are slightly more specialized, like the personal aegis, or "paling", and the spellblade (I swear I was gonna talk about these eventually!). These two are the most common in the specialized category, and are among the first things that children learn how to conjure in the Red Seminary (ordinary soldiers have the capability to learn these as well, but it would be much more difficult for them). The paling is a small ethereal shield (which covers your whole body, but only yours) that an Inquisitor can conjure at will. Depending on how powerful a warpriest makes it, it can withstand anything from bullets to actual spells from other magic users. Even the weakest paling can stop bullets and melee attacks from common soldiers, but when fighting against a rival inquisitor, a stronger one is needed. Think of the paling as a suit of armor that negates damage, but shatters after taking too much. Real talk? It's what makes it so that we can have cool magical battles without the combatants dying immediately in this RP!
Pieces of armor can also be enchanted to continuously have a magical paling, like Stina's shield and helmet, but repairing them and re-enchanting them after they become damaged takes a decent amount of time.
The spellblade is a streak of ethereal light made physical and honed to extreme sharpness that a warpriest can summon and then wield as a melee weapon. Spellblades are considered an Inquisitor's traditional weapon (the crest of the Red Seminary itself is a spellblade and paling interlocked within a red circle), as it is what the inquisitors of antiquity used to wage their battles. These days, inquisitors typically choose to fight using their own chosen weapons, and only resort to using their spellblades in battle if they lose their other armaments. Spellblades are also typically used in ceremonial duels, as it most strongly symbolizes an inquisitor's power. For instance, the strongest inquisitors can summon spellblades that can easily cut through the strongest paling.
For a trained inquisitor who has achieved a certain level of mastery in etheric control, spellblades and palings don't require all that much ether to summon. While they do require a level of intense concentration to keep them manifested, it is no difficult thing for graduates of the Red Seminary.
Now, Inquisitors are the only members of the clergy who fight in battle, thus out of all of the thousands of priests serving the Church, there are only about 300 active inquisitors (not many survive the Red Seminary, after all) in Varya. Only inquisitors use Omestrian ether to power their weapons and spells, the soldiers of the Secular Army are not allowed to use, and don't require, such a potent source of energy. As I mentioned before, most SA soldiers are conscripts who wield rifles that work with general ether-- the white kind that is harvested from animals and can easily be synthesized-- and armored vehicles like tanks use this as well. So, basically, regular SA soldiers are trained in ether manipulation and get by on their own skills, while Inquisitors are basically nigh superhuman death machines who also rely on ether manipulation (though it is more specialized) but also require Omestrian ether now and then to unleash their most powerful attacks.
Moving on to Ziotea's ether draining ability.
"I agree that Ziotea couldn't /keep/ any ether she drains. She has to release it, and she has to do so immediately. That means she can't just sap someone's totem and leave them dry unless she can release the resultant force somewhere, and she's not going to be able to control that force well (if at all). However, if she just wants to blow something up? There's no reason why pulling ether from that sort of place wouldn't work."
Agreed on that last bit, but when did I say that she couldn't keep the ether she takes? As long as its the right type, she'd be able to restock her pool with it. If I said something otherwise before, I'm sorry! I probably got confused.
"Given that everyone in Varya uses ether harvested from slaves (who all must have a type, and they aren't all Omestri because we know traitors and really bad criminals end up in there too), and that there aren't many Varyans of Omestri descent, this means that the ability to uses ether from another source isn't tied to the type."
Not everyone uses ether harvested from slaves. Omestrian ether is incredibly expensive and thus out of reach for most Varyan citizens. Most people use the white ether gathered from animals, which is the most common type and available to most citizens.
In regards to the bloodtype thing, let's go over the facts. An Inquisitor essentially has two pools of ether when they go into battle-- 1. their own pool, which is tied to a type (A, B, ect) 2. the Omestrian ether kept in a number of vials on their person (also tied to a type), which is what they use to fuel their big magic spells. Ziotea can drain both types, but can only use her own ether to do this, so, this skill would fall under the "general ether manipulation" umbrella. This skill is also unfocused, so it takes currently takes a large amount from her own pool. Now, as far as I know (correct me if I'm wrong) she can drain non-Omestrian and Omestrian ether, put it in a syringe, and stick it in her veins (or, if you want a more elegant solution, maybe it seeps through her skin and directly into her bloodstream, it's fine with me). The only way that this would actually restore her own depleted pool is if the new ether being brought in is of her own type. If it was any other type, it would not work and would probably have adverse effects. Does this sound right so far? I think I have it right.
Anyway, I think the main issue is that you might've forgotten about the whole "general ether manipulation" thing that allows soldiers and Inquisitors to fight without using Omestrian ether, (as well as the existence of white ether), which is what I think the rest of your post deals with. Now that I cleared that up, I think it should all work fairly well. Unless there's something I missed, please let me know if that's the case.
I was honestly confused about the whole ether tuning thing and didnt really understand it that well. I think it might be a bit overly complicated. I know the system I came up with isn't perfect by any means, but I think it makes enough sense. Again, I might've missed something here that makes it crumble all apart, but I think I explained everything well enough at the top of this post.