my one is a bit wired. I guess every dns server is different. In mine, I could only have cname for subdomains. So I added a subdomain called www just like yours and added the galaxy-ingress.meteor.com cname in there.
You should not be able to point mydomain.com with a CNAME record, I think that’s an invalid DNS entry. You should be using an A record that points to an IP address or a redirect (using other tools)
Well it is more intricate than that, but basically no, you cannot use a
cname for your domain root and have a separate record for www.
What you need to do is set up the cname record for www, delete the
cname record for the root, set up a www redirect using one of the
suggested ways (re: the galaxy dns help article)