The problem with this line of reasoning is that the way the GP operates, the only selection over which a host federation has complete control is the HOST PICKS. The top 3 seeds are allowed to name their choices, in order, and these are usually honored, so the hosts don't select those. After the top seeds,a separate draw is held for each category: seeds 4-6, Worlds 7-9, Worlds 10-12, top 24 WR, top 24 SB, etc. In each category, the host that gets first draw has the widest choice; the host that gets draw 6 gets what's left over. If a host always gets first draw, sure, they could cherry pick the best lineup for their skaters, but that is unlikely to happen.
Mirai usually gets invited by NHK, which makes sense, but it isn't always to her advantage since the Japanese ladies are a tough bunch, and the JSF usually uses the host pick for their seeds. USFS has no control whatsoever over invitations to Mirai or Ashley from other hosts.
You may be more knowledgable about the GP selection process than I, but the fact remains in 2012 SA was probably the weakest of the GP's, enabling Wagner to score an easy win. The field was so weak Gao was able to piggyback on the weak field and score a silver. You may say its luck or coincidence, I think the US Fed played a part in paving the way for Wagner to go to the GPF. Some Feds appear to want weak fields to help their skaters score points, other Feds may want stronger fields to sell tickets and not lose money as a host. In 2013, SA was still one of the weaker GP's, with Wagner getting a silver. Its much easier to have a "better body of work" when you Fed gets comparative cupcake GP's for you. TEB was also a weaker field for Wagner. You notice Gold, despite skating pretty well, didn't make the GP final. She had the difficult NHK, like Mirai. Gold also had a tougher field than Wagner at SC while Mirai had a difficult field in Russia. Mirai has a GP medal each of the last four years, more than Flatt, Czisny, Gao, Zawadzki, hicks, Cesario, Gold, Edmunds, etc; only Wagner has more.
Actually, Mirai has been exiled to Coc four times, an assignment which seems less popular. She has been to NHK three times, but the 2012 appearce was as a sub, when the US Fed only gave her one GP despite having a SB in the top 15. The NHK rescued her with a last minute assignment, after her own Fed left her hanging.