I'm just leaving a note. I'll edit this to be more complete when I actually study this.

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Please feel free to correct/add to this in the comments. I won't be getting back to this for a long while since it's in the range of N3-N2 and I'm still preparing to do a thorough review of N5-N4.

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--- Some hints / breadcrumbs for related grammar points to look at

All the below pages are from A Handbook of Japanese Grammar Patterns for Teachers and Learners.

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This is from the construction section for ではないだろうか on pages 286-287.

• N／Na (なの)ではないだろうか
• A／V のではないだろうか

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ではないだろうか (heading 4 of だろう) on page 235. Construction doesn't include (なの)

ではないだろうか on pages 286-287. Construction includes (なの)

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ではあるまいか on pages 280-281. Construction includes (なの)

ではあるまいか on pages 574-575 (heading 4 of まいか). Construction includes (なの)

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じゃないだろうか on pages 154-155. Construction includes (なん)

んじゃないだろうか on pages 702-703. Construction includes (なん)

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ではなかろうか on pages 287-288. Construction includes (なの)

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ない(か)〈mild assertion〉 on page 395. Potential replacements include the majority of the above expressions and more.