Definitely Andrew Scott's voice, I've decided.
I've seen him in Sherlock loads of times and those who have, will agree with me. He frequently changes the pitch and tone of his voice when shrieking away as Moriaty.
My issue is, its the changing pitch that makes it sound like Scott, but its with a german accent and I don't see Mendes saying for Scott to just be Moriarty, but german.
I'm sorry, but where do you spot this German accent in the middle paet of the quote? As a continental European, I know my German quite well but this doesn't sound particurlarly German to me. And again, if you have watched Sherlock, you know AS when you hear him.
Besides, CW dubbing the whole line in the German trailer is not the point, nor proof that in the English trailer it is CW as well.
To my opinion, it's definitely AS we're hearing in the middle part, with two possible explanations:
1. Mundane: they needed to fill a 'gap' in CW's quote for spoiler or timing reasons and decided this AS quote was fitting (you don't SEE CW during the AS part, which makes it slightly less messy). Also, this explains why CW is dubbing the whole quote in the German trailer.
2. Less mundane, and I admit less probable, but nevertheless very exciting if true: it's a clue. Mendes said earlier this is supposed to be a mysterious film and fans have to pay attention. Especially the quote itsef points to this: while we have been focusing on CW as the main villain, it has been AS who we came across so many times, yet we never really saw him.
I find this scenario not necessarily very probable, but nevertheless not impossible. Indeed, CW is a big, expensive name to add to the bill for multiple films. And I think, if this scenarion proves to be true, he will still be the main villain of SP. And perhaps even a second film. But, as Mendes has pointed out as well, the solution they found for Blofeld is a very out of the box-one. Why not have AS ready as a mysterious figure, operating from the shadows, only revealed as mastermind in the end, or possibly even in the next instalment? Everybody is focusing on Blofeld Pleasance/Savalas style, however bear in mind that in the favourite films of Mendes portray Blofeld literally anonymously, covered, with a mysterious voice and appearance. Wouldn't suit this the Mendes-style far better?
Anyway, to cut a long discussion short: anyone prepared to bet a nice bottle of Dom Perignon on who delivers this critical line in the film?