It is actually explicit in the OT:
“You, my friend, are all that’s left of their religion.” Sure, he’s talking about the Jedi there, but Vader is a Sith and he still talks about him in that way. He’s basically saying he’s the last force user. And Tarkin is a high-ranking individual in the Empire!
Nope. I think you explained yourself why it’s not explicit.
Nope. I think I explained myself why it is in fact explicit. I clarified that “religion” is in reference to the ability of using the Force, not the misconception that he meant the Jedi.
There are other canon sources which prove my point. In the novel Tarkin, he has a suspicion that Palpy and Vader are Sith Lords, but he doesn’t know for certain nor does he want to ask any questions regarding it. In the novel Lords of the Sith, Palpatine tells Vader that he “can’t be seen using the Force” to get them out of a sticky situation.
Of course, you probably don’t care about what canon says. I’m just trying to point out to you that Tarkin’s line can easily be interpreted that way, and is in fact interpreted that way by canon.
In conclusion, I would highly advise removing Merry’s line which suggests that everyone knew about Palpatine’s Sithiness in the OT, but it’s ultimately Hal’s decision.