Heading out to Moab in September and in preparation I am considering sliders and skid plates. Is this sufficient armor paired with a 3" lift and 285's for both these trails?
You don't need any of those things to do those trails. You will drag your trailer hitch a good bit.
You probably don't need anything for either trail, but the descent back to Sand Flats Road from Hell's Revenge once you get beyond Tip Over Challenge is all broken rock ledges, not slickrock. You need to take your time through this section without sliders or skidplates.