Did Mahan expect one war-winning battle?

Did Mahan regard a single decisive battle as the likely route to final victory?

No. He saw battles as part of gaining sea control, while the wider war turns on follow-on operations, recovery, and the opponent's capacity to continue.

Reducing Mahan to one war-winning battle flattens his treatment of campaigns and repeated actions across more than a century. A battle could be decisive for six months, a year, or two years while an opponent rebuilt. Its larger effect depended on what the winning force could do afterward. Mahan's emphasis on building a fleet also shaped how much attention he gave to the subsequent use of sea control. The expectation that one climactic action settles everything owes more to cinema than to this reading of naval history. Even famous decisive engagements rarely gathered every ship belonging to a major power.