I would again throw Mike Embley's name into the fray. As noted, he is adored by US viewers. Additionally, I now recall Mike saying on Twitter about six weeks ago that "more changes were imminent" for the America slots. (PBS viewers cried foul when Mike was removed from his slot, and those viewers instead got another edition of WNA plus extra hours of Babita Sharma and Adnan Nawaz, although they have also subsequently warmed to Babita in a big way.......)
Furthermore, it is quite strange to state that Mishal Husain doesn't occupy a "senior" role in BBC News. Whilst in a parochial sense, BBC UK (1 / 6 10) may be considered high profile, it is nowhere near as far-reaching as BBC World is. Put simply, there are far more people in the world who would recognise Mishal ahead of Huw Edwards, save perhaps in Leeds or Cardiff or Glasgow. So, who would viewers of WNA prefer to have as a replacement for Frei? Edwards, Husain or Williams? Of course, the answer is Husain, because they don't have a clue who the others are.
Apart from Embley, other possible contenders could be Alagiah, Willcox or even Edwards himself.
World News America is a step backwards for Alagiah and Edwards, and Willcox is still quite low down in the pecking order, plus he can be quite annoying at times.
I very much doubt Mishal Husain will move, and we know Sian Williams will not, in both cases they have young families.
IMHO if Mike Embley will move to Washington, he will get it, especially considering he is already established to PBS audiences, and it's a better gig than the aweful one he's had for the last few years presenting 2 bulletins on World.
If they cant get anyone from inside the BBC there are plenty of British journalists working in the states with a lot of presenting and reporting talent.