A couple of things and then I'm going to leave you alone because it's obvious that the reasons you do things have nothing to do with game play, but are driven by thematic, role-playing visions that you have in your mind.
1. You are not a tank. You do not have taunt, your barriers do not gain any sort of advantage from mitigation making barrier intrinsically weaker than guard. No matter what you do, unless you play on an easier setter (maybe normal, I don't know. No advance review copy for Varus), you'll likely still need a real tank in your party. Your "tank mage" is redundant.
2. Specializing all of the barrier skills onto one character just means that your build finishes slowly. If you spread the "pain" out across a second mage then you can gain access to all the barrier goodies that you prize so highly (for whatever reason) so much quicker. And since, as I tried and failed to point out earlier, you have no means of extending barrier duration beyond that which all KEs have access to (and will take) then staggering barrier casts is your best bet for max up time anyway. Additionally there will be times when barrier is needed for someone other than you (that tank I mentioned, or another party member about to bite the dust). A spare barrier in those instances is huge. So "dragging a mage NPC around" is in your best interest. Suck it up, you have 3 to choose from.
So there you have it. Two excellent suggestions for you to disregard on why a 31 point build focusing on barriers is a bad idea. Having said that I encourage you to do so because you'll probably enjoy it more and it's your game. I only argue with you for the benefit of the impressionable youth who might be reading, lest they be lead astray.
They are not a traditional taunting tank, but you can be certain that the are the best at mitigating the damage, barring the Champion because of Walking Fortress, unless Walking Fortress is usuable by Mages with staves that proc it, as we have indeed seen such one.
Tank generally means being hard to kill when someone focuses on you. Tanks in pvp don't taunt. The multiplayer pve traditional tank, is what you refer to, which happened long after the term tank was even used for D&D.
You have a wide array of abilities, that enable you to chain cooldowns and abilities, making you near invulnerable. You might need to do damage to grab attention, instead of a button that just taunts for you, but much more rewarding as a play style, rather than spamming taunts.
You have stronger barriers than anyone. Each time you do damage, besides the cd,, a barrier appears, and the more you do, the stronger your barrier, which reflects damage. When it's over, your opponents are frozen. With an ice wall near, or a simple frozen opponent, because of a barrier break, your base damage reduction is increased by 50%, just until you can chain your next cd. Then you have, fade cloak, with a rampaging damage on end and another barrier, and stasis lock. Besides all the CCs you can throw, or the Storm passive that paralyzes people around you, or Guardian Spirit.
There is no end to the cycle.
You can play with your real tanks all you want, there are people among us that don't need the playstyle of mash a taunt to grab attention. There was never such a thing in D&D games anyway, or Baldur's Gate. If i cared about taunts, i would be playing MMOs.
I think it is not an engaging play style at all, pressing a button to aggro everyone. Do abilities that produce more threat, or damage, and grab attention, or grab it by proximity. Or Block the road to your ranged, so they are forced to pass through your front liners. That's tactic, not pressing a button.
DPS Mages from afar, can take the -50% threat talent.
Rogues can drop aggro with stealth.
Warriors that damage can taunt, or not taunt if they want to avoid attention.
You are going to be doing either more damage than anyone else, if you looked at the trees, or they will be able to drop threat.
Vivienne was facetanking Dragons. Hint.
So, technically, you are not the best at grabbing attention, since you have to work for it. You have everything going for you, however, when up close and personal, as far as defense goes.
Knight Enchanters are defense and protection experts. You don't need personalized or party defense if you're not gonna be grabbing hits at all.
Hybrids that use magic and heal/absorb, will always be durable as heck. And if Walking Fortress is an ability you can put on a Staff, that gives the proc to a Mage, yeah, forget it.