Nothing's really broken or missing, I just want to bring up that the grammar page for のではないだろうか might benefit from some attention:
1. The highlighting of the example sentences is a bit off from the construction/grammar. I put a yellow line in the screencap below to illustrate this. Maybe it was purposefully done to highlight according to the translated sentences?

2. The other places that I'm looking have (なの), parentheses intentional, available as well as の.
From page 286 of the Handbook, the constructions are:
N/Na (なの)ではないだろうか
A/V のではないだろうか
It's also tangled with the usual piecewise alternatives like ではない to じゃない and の to ん, but I don't know enough yet to say if the other versions are strongly related enough for it to matter since this is related to indirectness.
--- edits (multiple edits resorted for readability)
Here are the related grammar pages I found in the Handbook
ではないだろうか (heading 4) on page 235. Doesn't include (なの)
ではないだろうか on pages 286-287. Includes (なの)
ではあるまいか on pages 280-281. Includes (なの)
ではあるまいか on pages 574-575. Includes (なの)
じゃないだろうか on pages 154-155. Includes (なん)
んじゃないだろうか on pages 702-703. Includes (なん)
ではなかろうか on pages 287-288. Includes (なの)
I'd link up any renshuu pages on the above, but I'm not far enough in the spreadsheet yet.