2-piece rotors, bolt + threaded hat, bolt+locknut or bolt+special nut?

[owdlvr]

The ideal would be to use a floating design with a specially designed floating nut or bobbin. Otherwise, go with a threaded hat like your top example. The only downside is possible damage to the hat with difficult to remove hardware (ie you strip the head and have to drill out the bolt). If this was a concern I would look at using a special step nut like the lower unit, but as you are using fixed rotors I'd save weight and thread the hats.

Elliptical offset nuts (all-metal-locknuts) are fantastic in some applications. For use on a brake rotor, however, I'd want to ensure aircraft-quality pieces, where the temperature and tensile strength are matched between bolt and nut. This is the only way to ensure they are truly reusable. For the cost, hassle and weight, just thread the hats.

Loctite 290 or 242 on the hardware, and use a belleville (domed) washer under the bolt head.

-Dave