Re: An API is not a triviality
The opinion of the Solicitor General is no stronger than any other informed opinion. Plenty of people think an API is trivial, as far as copyright is concerned. And the Solicitor General has some incentive to side with CAFC regardless of legal subtleties.
Also, the SG's opinion is not necessarily the opinion of the DoJ as a whole. It's officially the position of the Federal government for cases before SCOTUS and amicus briefs filed by the DoJ; but the SG is not the only lawyer in the DoJ, or even in charge of the department.
And the appeal to Microsoft's actions is irrelevant, even if your summary of the case and assumption of their motives are correct. They might simply have decided it was more cost-effective to pay Sun.