Not criticizing you or your engineer, but I don't think I'd have been satisfied with your solution.
The original mounting point has a captive nut on the other side of the steel, THIS is what supports the load on the seat, a thread cut in the rail is much easier to tear out than it is to pull the captive nut through the rail.
Also the rail is not thicker than the diameter of the tread it is supporting, so that's unacceptable in my view.
The rule of thumb to work with is thread diameter +20% for thread engagement. This makes the BOLT the weakest point; not the thread.
If I was your engineer I'd have made it a requirement to replicate the captive thread BEHIND the rail rather than tapping a thread into the rail.
You could correct this by sliding a nut in and tightening, or using a tapped steel plate 10mm thick instead of a nut.
Either that or I'd have required a flat bar rail between the original mounting points (10x40mm) and then new tapped holes in that to pick up the rail mounts. Countersunk high tensile screws would keep everything flush on the underside of the 10x40mm intermediate rail.
I've been fabricating for over 20 years now, and OVERKILL is the only way to go on critical parts.