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Try entering your @ handle in the Verification field. When you save it, it'll convert it into the URL, but it'll also make sure the fediverse:creator tag is there.

Yes, it's probably due to not having a full-size preview image. If you're a Pro user, you can also upload a large default image under the "Social Image" customization section, which should help. This is entirely on Mastodon's end, as far as whether or not the attribution shows up.