Yes, remove the one with an A record; use CNAME instead.
Just enter your domain in the input, then copy-paste the whole output into your app.yml config file, as I showed you in the screenshot! Be careful with the indentation!
Are you sure Chrome doesn’t automatically remove www, which you overlooked? I don’t see other explanations.
No, no. Discourse uses Let’s Encrypt! Sorry for the confusion. That’s why, you should use it, it’s free and easy.