The skull is actually INSIDE the faceplate, theyre two separate things. Go watch a video of the faceplate opening in slow motion and you'll see.
And it would be prohibitively difficult to make a reliable opening mechanism to open the faceplate, the mechanism would be too bulky to hide effectively.
Just my 0.02c, but i dont think a mechanically opening/closing glowing skull promethean knight helmet is possible if it isn't WAYYYYY too big for the head.
+1 to OP, interested to see the rest of the build. Helmet looks good, can't wait to see if the rest of the armor works out. Scaling on this suit is a bitch.