Is Mahayana and Vajirayana FanFiction?

I am guessing they are.

I believe I am an orthodox, exegetical, classical theravada buddhism lay person.

May all beings know the sukha of Nibbana, which is feeling nothing at all.

As CT we often say that those teachings that match are true and those that do not match are not CT. Other forms of Buddhism are another religion in my mind. However, Venerable Maggavihari seems to disagree. He agrees that by vinaya standards in terms of who is a monk, they are not considered to be a Theravada monk.
However, by terms of going off to another sect or religion without renouncing first, he believes that it is okay for a Theravada monk to mix and match robes and practices without the penalty automatically disrobed. That is his idea. It happens often with Mahayana who join Theravada and do not renounce their mahayana ordination before hand.