Not available for me because I live in Spain. Why, oh, why... T.T
Color subsampling doesn't affect DR, at least not in a meaningful way. The a7S in 1080p 420 has around three stops better DR than the GH4 in 4K 422.
I expect that the DR of the a7R II will be on par with that of the a7S, because both use the same gamma curve (slog2).
4K prores 422 HQ 24p is 750 Mbps, too high for SD and very expensive indeed on CFast (22 minutes on a 128GB card). CFast prices should fall really fast now that they're becoming more common, though (it's just CF with a SATA port instead of IDE).
You're thinking of the NX1
Great to read about your experiences with CENTS.
YOU MADE A MOVIE!!! (almost)
Let's all give Canon a big round of yaaawwwwnnnn