"She is coming to swim once."
Translation:Zij komt een keer zwemmen.
She is coming to swim one time or another.
She is coming to swim some time or other.
She is coming to swim on one occasion or another.
She is coming to swim at some point.
We are stating only that she is certainly coming at least once. She may indeed come more than once, but that is not what we are saying. We are stating that for certain she will come at least once.
Well technically you are right. since there is not always a rigid distinction between 'een keer' and 'één keer' (note however that while spoken there is a very distinct difference in emphasis!)
So, 'Zij komt een keer zwemmen' could mean either 'She will come to swim sometime' or 'She will come to swim one time'.
And 'Zij komt één keer zwemmen' could only mean 'She will come to swim one time'.
It is hard to say why not, but it just isn't. Grammatically it does not make sense.
You either omit the 'te' as in "Zij komt een keer zwemmen" (She will come some time to swim) or add 'om' as in "Zij komt een keer om te zwemmen" (She will come some time to swim). The second is more a statement of purpose (the swimming) while the first is just describing the action.
While this sentence Dutch > English is a good example of teaching students about a Dutch way of conveying an open-ended intention and/or a similarly casual arrangement for the future, the above English > Dutch practice sentence does not work at all. (What? Only once? Why?!?) In Dutch, this (a verb, in present tense, followed by "een keer") is a standard way to express a vague intention/arrangement for the near to medium future.