1) I think the rules state that the STB is allowed, bolt-in, from top of strut to top of strut. So, yes, the cross-braces make that one illegal, at least for STS. I'm not sure if that still applies in STX, and for SM, it might be no problem.
2) The rules say things like "must retain stock seat belts" but that doesn't mean you can't use additional harnesses. You just can't take the stockers out, in some cases. Again, I'm not clear on SM, but I think the car has to remain "street legal," and that would (I think) be the driver for retaining the stock belts.
