ないものでもない

1. 1Expresses reluctant or conditional willingness: "I might (do it), depending on circumstances."Ex. 1
2. 2Indicates that something cannot be completely ruled out: "it is not impossible that…"

A somewhat stiff, formal-sounding expression found mainly in writing and careful speech. It can follow a condition such as 〜なら or 〜次第では, as in the examples, signaling that the speaker is open to the idea only reluctantly or under certain circumstances. Compared with なくもない or ないことはない, it sounds more formal and roundabout.

• ないことはないN3「ないことはない」 has a similar meaning and is largely interchangeable with 「ないものでもない」 when attaching to verbs. However, it is neutral in register and can attach to adjectives and nouns, often followed by 「が」 or 「けど」, whereas 「ないものでもない」 is more literary and carries a reluctant-concession, condescending nuance.
• 〜なくもないN2「〜なくもない」 is similar in meaning and mostly interchangeable with verbs, but it is neutral in register and milder in tone, with weaker reluctant-concession or condescending coloring. It also attaches more broadly, e.g. to adjectives or 「noun＋が」.
• なくはないN2「なくはない」 also expresses qualified affirmation, but it is neutral in register and can attach to verbs, adjectives, and nouns, often followed by 「けど」 or 「が」 as a contrastive. Unlike 「ないものでもない」, it lacks the reluctant-concession, condescending tone.
• わけではないN3「わけではない」 negates an inference ("it's not that...") and can attach to verbs, adjectives, and nouns with a neutral register. 「ないものでもない」, by contrast, is a reluctant qualified affirmation that attaches to the ない-form of verbs, is more literary, and generally cannot be swapped with it.

ないことはない, なくもない and なくはない express similar partial affirmation but sound more conversational, while ないものでもない is stiffer and typically stresses reluctant willingness. わけではない differs in function: it denies an inference or assumption ("it's not that…") rather than hedging willingness or possibility.