I like what you sai there, though I do have one or two polite observations
I read on here once, that spot reduction occurs through hormonal changes. I also read once somewhere else, a former figure competitor, saying it happened in her Biceps through a combination of giant sets and Cardio.
I choose to give both intepretations of spot reduction some creedence, but I would say that in general, spot reduction, if not 100% impossible, is certainly very difficult, no matter how you train.
The thing you mentioned about training As in relation to a bad back, is something I agree with up to a point, but I think it depends on the problem.
If you have a mild twinge, or a bit of stiffness, Abs training shouldn't worsen it providing proper technique is observed, but if you have something like Fibromyalgia or Ankylosing Spondylitis, or Scoliosis etc.; basically anithing that causes significant pain or deformity to bone and / or muscle, than it would depend I think, on the levels of the backs tolerance, relative to the type of exercise(s), performed on the Abs, from direct work or indirectly from compound exercises.
I would perceive planks for example, as okay for people recovering from the remnants of some serious back trauma, or in possession of a chronic back complaint, but I wouldn't necessarily think that of something like Cable Crunches, or the indirect exertion from an exercise like good mornings.
Though any well executed exercise on an agonist muscle, is a strong likelihood to be beneficial and / or safe for antagonists yes