You can make probabilistic arguments about codon usage all you like, but you are missing the elephant in the room. If it was an engineered virus with a few changes, then the parent virus would be well known. We don’t know about any such beast so spew your conspiracy theories all you like.
As far as which ARG codon is common in coronaviruses, it depends on the virus and the host. The virus would adapt to the host and we don’t know the natural host (i.e. the animal reservoir) so your probabilistic argument is meaningless.
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