WordPress multisite with multiple parent domains

Like C C said some clarification would be helpful. If it helps, you can actually still use a subdomain and point the primary domain to it (the subdomain) using a flattened CNAME record. It is RFC compliant and functions the exact same as pointing the primary domain to an IP using an A record. Maybe this mitigates your problem. This article by CloudFlare explains it further: CNAME Flattening by CloudFlare

This is a free service by the way.