"I had declined using iDrive due to backward compatibility limitations."
Me too for several months. I have an iMac which has music software which is not compatible with Yosemite. But I updated to iOS 9 last weekend on my iPhone 6 & iPad 3 because I am getting an iPad Air 2 which would have iOS 9 on it and figured I needed to make the transition on my old iOS devices.
"I decided to see if backing up to iCloud would restore the missing documents. Unfortunately, it didn't."
That's right. After the iOS 9.3.2 update all Pages documents were gone on my iPad/iPhone and there was no quick way to bring them back. A big problem since I constantly use Pages.
"Luckily, I haven't updated my iPhone and all my missing files are on it."
I didn't check Pages on my iPhone after I updated my iPad to iOS 9.3.2. So, I also updated my iPhone too to 9.3.2.
So, when I checked Pages after going to 9.3.2, both iPad and iPhone Pages docs were gone. And there was no quick way to get them.
* I surrendered and updated to iCloud Drive on my iPad 3. My iPhone updated to iCloud Drive automatically after the iPad change.
That brought my Pages docs back. That's the good news.
The bad news is;
1. It is a one way process. No way to go back to regular iCloud.
2. Because my iMac is not running Yosemite, Pages no longer syncs between my iPhone and iPad.
So, iCloud Drive got my Pages docs back but iOS Pages syncing is dead (updating gets stuck).
I called Apple Support and they could not fix the problem. Basically an iMac with an older OS with the same Apple ID will stop iOS iCloud Drive Pages syncing on linked iOS devices. Notes works fine but not Pages.