My thoughts are as follows.
First, the Structure Target specifically states that: The spell affects everything within a single structure." As defined, a single structure starts at about 10 rooms, with extra magnitudes raising this by a factor of ten. Structure is a +3 magnitude modifier, with potentially more for larger structures. So there already are magnitudes added to account for the number of targets - three or more magnitudes. To add more to "tailor" the spell seems counter to the description of what the Structure Target covers.
Second, the collected wisdom said that you could only tailor a spell for a valid Target of the spell. If the spell has a Target of Individual, then you could only tailor it to a single person/item. If you wanted to tailor it to more than one person/item, you would need to invent the spell with +2 magnitudes for a Target of Group to cover those multiple people/items. It logically follows that if you invent a spell with +3 magnitudes for a Target of Structure (+1 magnitude above Group), that you should be able to tailor it for a valid Target of that spell, i.e., all of the people/things that could fit inside the targeted structure (keeping in mind that if I want the structure to be ore than about 10 rooms, I need to add magnitudes). Likewise, if I were to invent a spell with +4 magnitudes for Boundary, I should be able to tailor it to a number of targets that fit within that boundary. I'm already increasing the complexity of the spell to get the desired Target.
I fully understand that it's not a moving target, of course. As I said, if I want to tailor a spell to cover all of the furniture and grogs in my tower, then I need to pick a set of furniture and grogs (who probably have to actually be in my tower for some portion of the spell design) for whom the spell is specifically designed. I do not have the option to add any new furniture or grogs in the future without them suffering Warping. (Note: I'm not equating grogs with furniture. 
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Third, adding magnitudes for tailoring a spell invents a requirement that is not in the rules. No one says that you should have to add a magnitude to tailor a Target Individual spell to an individual. Why would you need to add a magnitude to tailor a Target Structure spell to the contents of a structure? You said in another thread that you were trying to stop adding magnitudes just because things aren't specifically described. Here you want to add a magnitude for something that is described, and is described such a way that it shouldn't require an extra magnitude.
Fourth, adding magnitudes for the 'complexity' of covering all the people in a structure is likewise redundant, You are already raising the spell by three magnitudes to get the Target Structure, which by definition covers all the people in a structure. Why would you need to add more magnitudes? It's like inventing a spell with a Target Group and then saying that you need extra magnitudes due to the significant complexity of all those people. That's what the added magnitudes for Group and Structure are already for.
Fifth, I didn't intend to say that Warping invalidates the Target Structure. I said that if you interpret the rules such that the you can't tailor a spell to cover some of the necessary targets of that spell then you invalidate the rule that you can tailor a spell to avoid Warping. Perhaps I was inelegant in the way I phrased my argument. What I means was that the rules specify that you can tailor a spell to prevent Warping. In my opinion, that necessarily applies to a valid Target for that spell, whatever that might be. In the case of a Target Structure spell, that would be whatever people/things in the structure that would otherwise be Warped when the spell is cast. If some portions of that target will suffer Warping even if the spell is specifically tailored, then I effectively cannot tailor the spell, and the rule that you can tailor a spell is invalidated.
Sixth, saying that you need a Breakthrough to allow structure spells not to Warp again reads a new limitation into the rules that is not otherwise there. The rules specifically say that "[a]nyone subjected to a powerful mystical effect gains a Warping Point, unless they themselves were responsible for the effect or it was specifically and carefully designed to work on them. ... Designing an effect for a particular target requires, in Hermetic terms, that a special version of a Formulaic spell be invented. The spell has its effect on anyone, but only the designated target, and the caster, do not suffer Warping unless the effect is also continuous." In this case, the designated Target would be a particular structure and a particular set of people/things that go inside of it (a valid Target under the rules). Why would you need to develop a Breakthrough to use this rule? You wouldn't need a Breakthrough to tailor a spell to an Individual or a Group; why would you need a Breakthrough to tailor a spell to a Structure?