Nope, you don't need to use a data layer in this example, it's all done through GTM. If you were tracking something else, it could make sense, depends on what you're trying to implement. What do you have in mind?

The example setup doesn't change your existing implementation at all, it just adds on an event tag. It's completely standard that UA fires on all pages, so don't change that, just add an event tag with a trigger for the page or event you're interested in.

Good questions, thank you for your comment! Sure, in this example case you could add the custom metric to a Page View tag instead of an Event tag. The custom metric would be implemented in exactly the same way and have the same result. I chose to show an Event tag example though because 1) in my experience that's a much more common use case, and 2) it keeps the Page View tag clean. In this example, if you sent the custom metric with a Page View tag instead, now you'd need to either write some new trigger logic using JavaScript, or duplicate the original Page View tag and then add blog-specific triggers and exceptions to both versions, to ensure that the custom metric only gets sent on the tags on the blog pages.

I wouldn't populate the Value field since there's no measurable monetary value to someone viewing my blog. I guess I could make an estimate for it but I prefer to keep Values for something more concrete.