How would the oarsmen-marine ratio of the male population square with the feudal levy one? ~54000 men is immense. And what about the ~1000 merchant vessels they supposedly have? Is Cersei (completely) wrong? I find it hard to believe the Redwynes have anywhere near that many men and ships …

Because oarsmen aren’t fighting soldiers, they don’t fall under the same rule of thumb. They don’t need arms and armor, they don’t need training in hand-to-hand, you just round up people who know how to pull an oar in time. And the Redwynes live on an island, so there’s going to be lots of them.