There is 1.5 s GCD on devastate (
https://wotlkdb.com/?spell=47498) . You can spam it as much as you want until you have rage. It's the same for sunder armor spells . The effect itself apply by devastate have a duration of 30 s on target and 0 CDG (
https://wotlkdb.com/?spell=48893).
Sorry to said that but no it's not simple to calculate exactly threat. At best you can do estimation because there are still some unknown stuff due to you touch mecanic of game. Blizzard are not really fan to show their formula for everthing. (If it's was the case all private server can be perfectly blizzlike). It's why you it's calll THEORYcraft. It's made by fan who try to extrapolate as much as they can.
The data you find is raw data for spells without taking care of talents, procs, buff, race, rage gain, weapon speed, the diff between normal hit crit hit blank hit..... The way of all of it's mix together is unknown (it's the blizzard cola formula). Where and when exactly you put multiplier exactly is unknown. Even some multiplier are still unknown.
And if you ask why addon like Omen can do it. If you open source code of this kind of addon. You 'll find 0 formula. It's works because this soft call a particular blizzard librairy where everything is. But you can't have acces to this file.