It was as I expected. The version in the question doesn't re-launch the app, because /usr/bin/open doesn't return an error status when the app crashes. I wasn't able to find a better way than hard-coding the path to the executable, so I'm now using the following.

<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>KeepAlive</key>
        <dict>
                <key>Crashed</key>
                <true/>
                <key>SuccessfulExit</key>
                <false/>
        </dict>
        <key>Label</key>
        <string>com.getsync.bittorrentsync</string>
        <key>Program</key>
        <string>/Applications/BitTorrent Sync.app/Contents/MacOS/BitTorrent Sync</string>
        <key>RunAtLoad</key>
        <true/>
</dict>
</plist>

I've verified that this does successfully re-launch the app if it crashes.

A more easily downloadable version of this can be found at https://gist.github.com/atomicbird/ff2de46a594cdb5c8e69

According to this Apple iCloud Drive support page:

I’m editing a document now. When will my changes upload to iCloud?

When you'll see your changes depends on the app you're using. Pages, Numbers, and Keynote upload files to iCloud regularly to make sure you don’t lose any changes.

Other apps upload your files to iCloud only after you save them to the iCloud Drive folder. To see your saved files on a Mac, go to Finder > iCloud Drive. On your iPhone, iPad, or iPod touch, go to the iCloud Drive app. On a PC with iCloud for Windows, go to File Explorer > iCloud Drive.

If you still don't see the latest changes or you see an "in-progress change" icon or the iCloud Drive icon in the notification area (system tray), try to save a new document to the iCloud Drive folder. Then check the iCloud Drive folder to see if the changes appear.