The Doylist argument against killing goes right out of the window if consider Force canon (and I don't see any reason not to), besides, it's not a very fun one. :P
Regarding your other points, you often argument from "infeasibility", but I do not think our thoughts on feasibility of certain notions apply to a highly-advanced futuristic society that exists across multiple worlds and has confronted and apparently long solved social questions that ours doesn't even begin to consider (like how to deal with living sentient beings who are not human (dragons), humans who have been grown in a vat, and sentient beings composed entirely of magic). Given, however, how this counterargument provokes the notion that we can't really say anything about the Mid except what has been directly shown to us, I will accept the validity of your feasibility arguments for now.
Now, we clearly have examples of Ancient Belkan magic being used to kill, and we know from canon (though I can't provide a specific citation) that both Belkan and Mid magic rely on the same type of energy/mana. Hence, if there is a difference in how the two systems treat living and/or organic and inorganic matter, it must be in their own restrictions of how magical energy is used, rather than in the nature of magic itself. My original argument was that most attack spells of the Mid school are non-lethal, even if taken without a Barrier Jacket. Conversely, there are examples when a person is physically harmed by a Mid spell even while wearing a Jacket (Precia shoots right through Arf in season 1, leaving a perforating wound; Nanoha's fingers bleed when she catches Teana's blade with them during the White Devil incident). With these and previously named examples, it appears to me that while magical energy can always be used to cause bodily harm, the vast majority of Mid spells are designed to overwhelm the target's magical defenses (if any) and nervous system instead. In other words, Mid magic is set to stun unless specifically designed to kill, which is strongly discouraged (remember that Precia was an insane wanted criminal, while Teana got head-cooled as punishment for, among other things, using a lethal form of magic).
The other point of contention seems to be whether TSAB is a military or not, and consequently, whether they need lethal weapons that can be set to stun or non-lethal weapons that need an optional lethal setting. My answer is still that they are not a military, since there is no other organized military or paramilitary force for them to fight, and we never get any indications that there have been major interdimensional armed conflicts since the Belkan Wars. As such, I cannot imagine which "external threat" you may be referring to. In such political situation it would actually be very easy for TSAB to position itself as the interdimensional law enforcement and administration body that only polices, without imposing its will with military might. Recall, for instance, that they do not interfere in the armed conflict on Orusea (SSX), even though, have they had an active military, they could have easily moved in and nipped the hostilities in the bud with superior firepower. Taking the Doylist perspective again, I think that TSAB is much more inspired by the bureaucratic structures of the Chinese Han and Tang dynasties (Han get a bonus for rising from the ashes of the Warring States, similar to how Mid rose from Belka's remains, while Tang pioneered the standardized public servant exams like the Mid has for its mages, see StrikerS), than by the pre-modern and modern Western (particularly British-American) imperialistic militarism.
I would also like to point out that what makes an organization a military is not the ranks, the weaponry, and the vehicles but the purpose and the personal training derived from it. A military's purpose is to destroy other states and militaries' combat capacity, allowing their respective rules to enforce their policies onto other states. A police's purpose is to preserve social order, using violence if necessary, but always with the goal of saving lives and property, if possible. While both require extensive physical training and discipline, one major difference is in the psychological conditioning: a military trains you to kill before you and your squadmates are killed, a police trains you to find ways to spare even those who try to kill you (unless, of course, you live in some crazy country where the cops always shoot to kill). Given that there has not been any comparable military or political structure for TSAB to fight in the past 80 years (see above), and that at no point do we see any TSAB personnel officially sanctioning the use of lethal force (and recall that Signum specifically declines training duties in StrikerS because she can only teach others to kill and doesn't consider it useful knowledge for Bureau cadets), I'd say it is much more logical to assume that TSAB sees itself and functions much more as a law enforcement agency than as a military. And as such, they should require their agents to use non-lethal weaponry as much as possible (even in the face of criminal using lethal weapons against them -- in fact, they would be specifically trained for that, just like the RL police), especially considering that most of their engagements must happen in densely populated urban areas and we know that even on the Mid, most people don't have magical talents and cannot summon Barrier Jackets (and Yuuno is apparently the only guy in the universe who is overpowered enough to actually stop and reverse time to fix collateral damage).
If you would like to know what the Mid would look like if TSAB actually was a military with someone to fight, I can recommend reading the Nanoha BetrayerS doujin. Or not, if you don't want to witness the Nanoha canon and ideals completely perverted and defiled.
