Not sure my initial reply warranted such a detailed worthy reply. Briefly:

1. The choice of stainless vs steel for the K, no matter what the use, was a poor choice. Which means if Stainless would work, a quality steel could also be used, in a thinner gauge.
2. My concern on flimsy seat supports centers on, a seat belt holds a driver against the seat, and in most cases, if the seat supports distort, the belts go slack.
3. I am not sure if part of the original calculation on your part, but a 3" alum exhaust with its greater surface area and thermal mass, vs say a 2.5" dia, may have been a big plus in the exhaust longevity equation, just sayin.
4. I might have mistakenly assumed the painted white crossmember was Alum as you originally proposed, is it steel?
5. My concern on TI, any application with reversible load cycles, or bending, which a wheel stud might ( OK its not supposed to), is a candidate for Ti fatigue, and the consequences very unfavorable, and hence my concern, because regular replacement at high cost is easily pushed aside. Steel is far superior to Ti regarding fatigue of course.
6. On Axle u bolts, is the flat spot that crucial? I'm sure I've seen u bolts without a flat spot. I have also seen housings dimpled. If manufacturing is that difficult, how about a 9/16" U bolts, and just machining off a 1/16"? from the inner curve, creating the flat spot, and still be stronger then a 1/2" u bolt?
7. "TB's" are my handling slang for torsion bars, sorry.