Slicer was so fast Edward didn't have time to do full transmutations (which consist in comprehension, deconstruction and reconstruction of matter), so he managed to do what Scar does (stopping after step two), which is faster (and deals a lot of damage).
11:12 The previous anime stated it expressly, however it makes sense also here: since it's the blood symbol which tethers the soul to the metal, that's the "main" part of the armor. If a part of the armor gets detached, it has got no connection with the main part and the soul cannot control it anymore. When Alphonse gets his head detached, his eyes stop glowing.
In the English dub they use “he,” but in the manga from what I could find they just say like “that person Envy” or “that guy” (I’m assuming “guy” is being used like “person” in this context as well, so nonbinary)
