So as far as the CGI Tarkin and Leia go – the Rogue One team actually did recast those roles, and not with nobodies: they had English stage actor Guy Henry as Tarkin and Norwegian actress Ingvild Deila (seen in Avengers 2) as Leia. It’s just they then poured CGI all over them to resemble Peter Cushing and Carrie Fisher circa 1976.

I’m certain an alternate cut of Rogue One exists which uses the original performances by these actors. It’s just currently in the Disney vault, along with the R-rated animated Willow trilogy and other things hidden in the time capsule created by George Lucas and Steven Spielberg.