I agree that using preferences in a single file (viewer.js) gives a better overview (even though "hunting for them in different files" is straightforwards with any decent text editor or CLI. `grep -r 'Preferences\.'`).

Your suggestion works for *getting* the preference, but it does not help at setting the preference (Hand tool can be toggled using <kbd>H</kbd> shortcut, or by clicking the button in the secondary toolbar). Because the preferences are currently initialized only when the PDF is loaded, it is not possible to reliably use the Preferences API outside that specific code block.