How Would SD React To Partial PLIST Upon Startup?

Is it possible to use a partial PLIST that would be fleshed out with defaults for missing values when viewed via the Preferences... menuitem?

Something like this “com.latenightsw.ScriptDebugger7.plist” placed in the
~/Library/Preferences folder.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>ApplicationHasBeenRun</key>
        <false/>
        <key>ApplicationLastVersion</key>
        <string>7A125</string>
        <key>LiteMode</key>
        <false/>
        <key>SUEnableAutomaticChecks</key>
        <false/>
        <key>SUHasLaunchedBefore</key>
        <false/>
        <key>kPrefSWReg70Registration</key>
        <string>000B0-09UC0-000RW-RQ806F</string>
        <key>kPrefSWRegUser</key>
        <string>John Doe</string>
</dict>
</plist>

Would opening the Preferences dialog augment these values with missing defaults, saving the merge upon closing?

Yes, that should work.