According to
Official Site, the OAD series will be 4 episodes. Though, I don't know how many DVD/BR box sets they are planning to make this season.
As for Amber lookalike girl, I initially thought she's very unlikely to be Amber. But I found something interesting in the video. Her lip's moving but there is no voice. Among all characters, only her voice is muted. Stragne. Well... Amber's voice actress has been ill since last year and she is coming back soon. Just coincidence? Or another commercially motivated trick to lure Amber fans? Who knows! Anyway, I will not ignore a possibility of real Amber (probably in Hei's flashback).