Robin, derate and assumed temp are two different issues.
An airline can order aircraft (let's assume the -800) with 27k, 26k and 24k power rating.
As said, the 27k is just a bump power, not used for every take-off and it has to be selected by the crew with the FMC.
The 26k rating is standard. You will find in the FMC a page with the thrust rating, the crew can opt to use only 24k for take-off; this option is only advisable for the crew if their respective airline allow for it.
The 24k rating is a perfect alternate solution for airlines with very light take-off weights, i.e. minimum catering, no holiday charter, low fuel loads. It applies to almost all low-cost carriers.
The assumed temp does not change your thrust rating, it just tells the computer that your environment is that of a hot summer day and thus reduces the fuel input. Basically that's it.
There has been a discussion recently about combination of de-rating and assumed temp procedures, again, this depends on your airline if they allow it, it is not recommended by most.
Back to the thrust rating. The -800 is certified for 26k thrust rating (that includes the 27k bump power). The engine has a rating plug with pins and the engineers set these pins for the rating your company has opted for. Since it is the very same engine for 24k as it is for 26k, this plug decides about the available standard thrust for this engine.
However, this is just the standard as the autothrottle will use it commanded by the computers. In case of fire-walling the throttles the pins for the standard setting in the plug are bypassed and gives you the fully certified thrust for this airframe, the mentioned emergency reserve thrust.
NSEU
as far as I understand the plug has two settings. One setting is the ordered thrust rating, as per the airline, the other setting is the airframe max rating.
The emergency reserve thrust allows for the airframe max rating, whatever this may be.
This feature makes the engines available to several different airframe versions and just needs the correct plug setting for an airframe change.
There should not be any safety concern as you will never get more thrust as has been certified for this particular airframe as long as engineers use the correct plug setting. Why should there be any concern at all, if an airline opts for 24k on the 800 and you need the 26k for evasive maneuver, there are other airlines which operate the aircraft with exactly this power in standard configuration?
Hope this helps