YouTube – Are Comments Upvotes/Downvotes Anonymous?

Update: You don't have to do anything.

The comments you make on YouTube will now appear only on YouTube, not also on Google+

The only YouTube activity that gets posted to Google+ is comments. To stop that from happening simply uncheck "Also share to Google+" when commenting.

What are the current rules/limitations about having one's Gmail name/address visible when one comments on a YouTube video?

The answer is found under: Control what others see about you across Google services.

Note: Your name and photo (if you choose to add one) will be shown to other users across Google products, including when you communicate or share content.

It is stated even more explicitly under your My Account-->Privacy-->Email settings.

The address used to identify your Google account to you and others. You can't change this address.

How can one add identifying email to a Gmail account without it showing up when one comments on a YouTube video?

You should create an alias for your account and use that one for your emails.
Please see the answer to this post How to remove Sender header when sending from another Gmail on how to do that.
Your only other option would be to create and use (in a similar way) a new Google account. The drawback to this approach would be that toy would need to log-in to separate accounts to send emails and comment on YouTube.