I'm definitely not an expert here, just a volunteer, but the assumption that SSL requires different IP addresses for each site seems off, or perhaps, outside of the concerns of multisite. The answer to your question, though, likely involves cookie syncing. If you have SSL on the multisite, visitors should be carrying a cookie in their browser that labels them properly. With Domain Mapping, the cookie is synced with the mapped domain as if the URLs were the same. This cookie sync is an important function provided by the plugin and why we don't encourage attempting to domain map without the plugin. Whether the new IP address needs to be addressed overtly in the multisite is not something I am familiar with. However, if this is vital, you should be able to take care of this in htaccess.