Flymesome.
We didn't have the option of choosing between Flaps 15 and any other Flap setting on our QRH in the One Engine Inoperative Landing Checklist. Does planning a Flaps 5 landing raise the question of legality in terms of crew training?
As I recall. you are right. 15 flap on one engine. However, there are times when a command decision calls for operation outside SOPs. That decision was made here, and I would suggest it was the right one. Successful outcome.