A complete different approach might be to avoid backend editing at all. Have a look at frontendcms.com, it is a WP plugin I wrote. It solves this and similar problems by giving the editor his/her own interface.

The probem of deleting widgets is more general, it is caused by the developer and the editor using the same backend interface. Similar problems like deleting widgets are

• inserting images in the wrong size
• adding fonts that were not intended by the designer
• creating more menu items than allowed .. and many more

The solution is simple, give editors their own interface. And probably the best way to do this is: let this interface look like the website itself. This approach is what I call "frontend CMS"

The frontend editor plugin is a free plugin that can be used for this. I myself wrote a plugin that takes this philosophy a bit further, frontendcms.com. It is not free however.