そうもないVery unlikely to, Does not seem that, Does not appear that

そう is used to express that something seems likely based on perception or indirect signs, e.g. 雨が降りそう ('It looks like it's going to rain.'). In contrast, そうもない (or そうにない) indicates that there are no signs of something happening, e.g. 雨が降りそうもない ('It doesn't seem like it'll rain at all.').

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.

そうもない and はずがない both express the improbability of something happening. そうもない is the less confident of the two and carries the nuance that something seems unlikely, e.g. 雨が降りそうもない ('It doesn't appear that it's going to rain.'). In contrast, はずがない is a stronger denial, with a nuance of disbelief or surprise, e.g. 負けるはずがない ('There's no way / It's highly unlikely [we] will lose.').

そうもない and わけがない are similar in that both express the unlikelihood of an event. そうもない suggests low probability based on observation, e.g. 雨が降りそうもない ('It seems unlikely that it'll rain'), while わけがない is an emphatic denial based on the speaker's logical conclusion, e.g. 勝てるわけがない ('There's no way we can win').

Both そうもない and っこない express unlikelihood, but differ in intensity and tone. そうもない implies doubt but leaves room for possibility, e.g. '間に合いそうもない' ('It doesn't seem like we'll make it in time'). っこない is stronger and more absolute, suggesting impossibility, e.g. 'そんなことできっこない' ('There's no way I could do that'). っこない is also more casual and colloquial, while そうもない is more neutral.

While both express impossibility, そうもない focuses on visual or situational cues suggesting unlikelihood, e.g. '雨が降りそうもない' ('It doesn't look like it'll rain'). ようがない・ようもない implies there is no method or way to perform the action, e.g. '連絡のしようがない' ('There's no way to contact them').

そうもない and そうにない are extremely similar, both expressing that something is unlikely or shows no signs of happening. The difference is that そうもない feels stronger and more confident due to も adding emphasis, e.g. '雨が降りそうにない' vs. '雨が降りそうもない' (same meaning, but the latter sounds more confident).