I've done some more examination of this issue, and here are my recommendations.
- If you want a user to have write access to Post Gallery items from the Dashboard, promote that user to an Author on the main blog. Authors will also have the ability to create new posts and pages (such as help pages), but given that it'll be a small number of trusted users, I don't see it as an issue. Note that Authors will not be able to edit or delete PGW items posted by others; for this, they'd need to be promoted to Editors.
- It's not currently possible to grant admin access to a single widget, or even to the widget page alone. The WP capability in question is edit_theme_options, which will give access to all theme options - surely more than we want to provide. So, for now, I suggest that management of widget-level settings continue to be handled by an Administrator. Seems like these settings don't need to be changed very often anyway, right?
As suggested above, we could skirt some of the structural limitations of WP's Widgets page by moving the relevant widget settings to a standalone settings page (where we can give access as we wish). That'd be an enhancement for a future release.
If this is all OK with you, then the following should happen: (1) you should promote the relevant users to Author/Editor, (2) we should open a new ticket for the further enhancement, and (3) we can mark the current ticket resolved for this milestone.