Can we talk about these busted bolts a bit more?
You substituted 1/4" socket head cap screws for the all-thread to start with. Since you were getting some racking, you kept tightening them and eventually snapped the bolts. Did you snap the heads off or did you snap them at the threads?
Why do I care? A SHCS should have been stronger than hardware store all-thread by a goodly bit. Not only that, but they should be stronger than the aluminum threads you were intalling them into. I would think you should have been stripping threads. Your heads went down into the countersinks, yes? Were the countersinks flat bottomed? It is rather easy to snap off the head off a bolt if you put it in bending like bearing it up on a non-flat surface. (But you didn't say whether it popped the head or not, so I don't know this to be the failure mode.) Might you have been using an inferior strength SHCS? Most of the ones I find are grade 5 or better.
Just curious.
I'm also thinking a shear plate between parts 16 and the spacers (and using thinner spacers) would get rid of the racking but I haven't verified that there is no other interference. Of course, now that your machine is working, you could just hog out a fancy replacement.