The opening verses don't clearly clarify and identify him as the Author in my eyes, they start like Paul called to be an aspotle, peace and greetings, to the chruch of so and so.
Why would Paul have ever needed someone to write for him, as it goes he was more than capable of writting letters with his own hand.
As you said in your reply "most likely" and most likely is not a fact, it's just an assumption.
If Tertius slipped in his greeting surely it would be obvious rather than just saying that he wrote the espisle.
I do appreciate and thank you for you reply and opinion, I will have a look deeper into it. If Paul wrote it I'm sure I will find out.