'Viewers' doesn't do what you think, and it is a deprecated feature that we will be removing in the next release. Just as an FYI, adding a viewer would allow that particular user to interact with your content without costing cloud credits. There was a global limit on the number of viewers that could be used to prevent abuse. Adding a viewer does not send any notifications which is why it seems to do nothing when add them. Anyway, that feature is being deleted due to lack of interest and the confusion it was causing users like you.

With collaborators, we have fixed things in the next release so that you can in fact add emails that are not current cloud users. Behind the scenes we set up a provisional account for them so that we can give them permissions to see the content. When the person you are inviting to collaborate completes the account creation process and signs in, they will see your document in the shared with listing. You can expect to see this improvement along with a significant improvement in the sharing dialogs in the next release which should be out shortly.

This is another thing we'll be making in the next release, but currently you can select Deploy->Published Page. From the dialog you can choose "Static Web Page" which will let users read but not interact (use dynamic content) with your content, This will not cost you any cloud credits. If your notebook has dynamic content like Manipulate's that you want your users to be able to interact with, choose "Interactive Web Page". This can cost you cloud credits (about 10 for every second of cpu time used) for the computation time needed to support dynamic content. The URL generated from this dialog can be used by anyone regardless of whether they have a cloud account or not since published pages are public and do not require sign in, so feel free to send it to whomever you like. In the current GUI there isn't an easy way to stop a document from being pubic, but you can do it easily using the Wolfram Language function SetPermissions in particular

SetPermissions[obj, "Private"]

where obj is a CloudObject expression with the URL to the public document will make it private again (removing all permissions for users other than yourself, the owner).

There is no way to enable Shift+Enter evaluation of cells for published views accessed by users who do not have a Wolfram Cloud product. When a user follows a published link (which use the www subdomain) he or she is given the option to open the notebook in his or her preferred cloud product using the Open In menu in the page footer. If this user opens the published notebook in a product view, e.g. Wolfram|One, he or she would be using his or her own account to interact with the notebook and would not be using your cloud credits. You probably don't want that user changing the notebook even if you do want them to be able to do Shit+Enter evaluation. If you set the {{AutoCopy}} option on the notebook to {{True}} but leave the notebook read only then the user will be prompted to make his or her own copy which can be changed without affecting the original. You can use

SetOptions[co, AutoCopy->True]

to do that. Alternatively, you can set the notebook permissions to All -> {"Read", "Interact", "Edit", "Evaluate"} (the Execute permission is for API's and not notebooks), so that the user can evaluate and make changes to the notebook but nothing they do will be saved. That's what we do with some of our sample content like the "Things to Try" notebooks we publish.