Depends on what you mean by “optimality”. Is it the best of all possible codes, or is it just a pretty good code compared to a randomly chosen code? Evolutionary biology would predict a better than average code.
Anyway, I don’t think you’ve thought through your own scenario. Were the seeded organisms designed from scratch? If the designers are organic beings from a word with life, why would they reinvent the wheel? They would just tweak existing microorganisms, keeping the code that’s already working for them.
But generally, they choose the solution that’s good enough, considering effort and expense. I submit that your designers would keep whatever code their life already had. Less trouble, less possibility of disaster in re-using an already-tested format.
It has been claimed that a former 2-base code can explain some features of the code we see. Would that be a “scar of history”?
It does if the precursors are suboptimal. Selection would dispose of them if they’re competing with standard-code organisms.
Not that I like Popper all that much, but I think you’re misconstruing him. Popper would want you to flesh out what you would expect to find if it was true but not if the alternative were true. “Silent” isn’t good enough. You need “expects the opposite”.
All fine when you aren’t talking about the origin of life. In your case, though, you’re merely transferring the problem to another planet. And as I mentioned above, the designers would most likely have used their own genetic code, so you haven’t even explained its optimality.