Reverse thrust is not normally considered in certificated performance.
Manufacturers ‘actual’ distances should specify the level of reverse assumed e.g. Boeing rev detent. There should be a separate landing distance table for pre-planned single eng reverse.
If you use 2 eng reverse, then it is the captain’s responsibility to consider a failure during landing; either eng failure or reverse not deploying. Although these failures might appear remote, the reason for certificated performance not considering reverse could be that the sufficient level system reliability was not proven.
Thus if a limited distance landing is based on 2 eng reverse, then the risk of rev failure has to be judged and possibly justified by the crew.
Also, consider what the manufactures’ landing distances assume; a long or fast landing without rev failure might not assure stopping within the published distance.