Most of my students login with Google to all the sites we use in class. Once they are logged into Google, then the embedded sites either recognize this Google login, or students can log into the embedded sites with Google from within the Formative.
If you have the students answer questions within EDpuzzle, there will be a time/date stamp in EDpuzzle with the answers. You would need to log into your EDpuzzle account to view the answers.
If you want to grade only within Formative, put comments within the EDpuzzle video that direct students to answer in the Formative. (Also - if you use this method, you can set the Formative to a public setting so that students don’t need to log into EDpuzzle at all.)
Two years ago I had to have students write down all their usernames and passwords. It was a hassle. This past year was awesome, because all they needed was to know how to log into Google. The nice thing about Formative is that you can make the Formative your ‘one’ place and then link/embed the other sites within the Formative.