> Google will not be able to receive any events/messages unless they're sent by not-google.com Exactly: the integration requires cooperation from both websites

> Google will not be able to receive any events/messages unless they're sent by not-google.com Exactly: the integration requires cooperation from both websites, and there is zero reasons, why child website would want to cooperate. Unlike, say, using Google Analytics, letting user leave you via <portal> tag is against website's best interests.