ない事(こと)はないIs not impossible, It doesn't mean that it isn't

はずがない strongly denies something, as in そんなことがあるはずがない ('That can't possibly be true.'). In contrast, ないことはない suggests that something is technically possible, but maybe unlikely or difficult, as in 行けないことはない ('It's not that I can't go.').

Both そうもない and ないことはない express uncertainty, but in opposite directions. そうもない shows strong doubt or improbability, e.g. '雨が降りそうもない' ('It doesn't look like it will rain'). ないことはない uses a double negative to express a less confident positive, e.g. '行けないことはない' ('It's not that I can't go'), which implies that while not impossible, it might be with some difficulty.

Both というわけではない and ないことはない introduce nuance or partial disagreement, but in opposite directions. というわけではない denies a conclusion or assumption, e.g. '好きというわけではない' ('It's not that I like it'). ないことはない uses a double negative to express a less confident positive, e.g. '行けないことはない' ('It's not that I can't go'), implying it may be possible but difficult.

Both introduce nuance and uncertainty, but with opposite slants. とは限らない expresses that something isn't necessarily the case, e.g. '有名だからといって面白いとは限らない' ('Just because it's famous doesn't mean it's interesting'). ないことはない uses a double negative to express a less confident positive statement, e.g. '行けないことはない' ('It's not that I can't go').

Both forms use a double negative to convey a positive meaning, but differ in nuance and usage. ～ない～はない means 'There is no (A) that isn't (B).', e.g. 書けない漢字はない ('There are no kanji he can't write.'), strongly affirming his ability. In contrast, ないことはない softens a statement, implying 'It's not that (A) isn't true,' e.g., 書けないことはないけど ('It's not that I can't write.', i.e. I can, but perhaps not easily or well.).