How would I allow guests to add comments and ratings to existent entries?
Using the Guest Entries plugin, it is not possible. Using a custom plugin is possible though. The trick is that each guest needs to be identifiable in some way. Otherwise every guest would be able to edit every comment, and you would have chaos (or a wiki).

So if you want them to be able to edit a comment and still not have to log in, you can create a custom plugin that replicates much of the functionality of the Guest Entries plugin. Adding on to that, you could create a unique string stored in a separate table linked to that entry. This string would be used to give the anonymous user a unique edit link. This would keep the comment reasonably safe.

Though in all honesty, you would be better served by having users log in or using an external commenting service (if you are avoiding buying a higher Craft version).