If the plane came from Boeing with 24k engines, 24k will be all you can get at the stops. If you select a 22k derate, you can still get the 24k at the stops, but you might VMCA yourself into the ground.
Indeed, and that is the reason why boeing advises against advancing the thrust levers if you have used derated thrust.
The only reduccion you can do, is by the assumed temp, which in that case you ll have less than 24k, and in this case you can changed to full power being 24kmax.
Not true. Usually you have three thrust settings available, full thrust, derate 1 and 2, in the case of the 24k thrust rated engines that is 24, 22, and 20k. If you use those you have a different set of performance data including different Vmca and Vmcg for each thrust settings. Additionally you can use assumed temperature on each of the reduced thrust settings as well.