With polygamy, the odds are not as bad as that, but eventually the species will still disappear.
However, the dialogue in "Elogium" never establishes flat out that the Ocampa would only be fertile once in their lives. The closest it comes to that is the phrase quoted above, and as said, it can be interpreted as meaning that the Ocampa females can only start producing children at this specific time; nothing goes against them continuing it thereafter.
Yes, but that's a very stretched interpretation. Kes says, "The elogium occurs only once. If I am ever going to have a child, it has to be now." A human girl wouldn't say, "I've started puberty. If I'm ever going to have a child, it has to be now," despite the fact that going through puberty occurs only once. From what Kes says, it seems that she can only become pregnant while she is going through the elogium, which only happens once. And given the way she says, "it has to be now," it seems that this is a temporary situation, not comparable to the several decades that Human women are fertile.