It's worth noting that Vanezis says suicides with multiple shots, whilst unusual, do occur.
I don't follow the argument that somebody wouldn't have staged her suicide with two shots, clearly it wasn't the intention, but what else could the murderer have done when the first shot didn't work? It also seems a little coincidental that there were only two bullets remaining.
Maybe there was only one bullet left and the killer had to go and fetch another one, stashing the silencer in the gun cupboard whilst he was at it. 

PRE-CISELY Hartley. I -and others- have pointed out this fact to Alias on sooooo many occasions. We KNOW it wasn't ideal. We KNOW it wasn't what he'd have planned. We KNOW he hadn't allowed for the possibility, but it ended up being, no more or less, what it was and simply stating that he wouldn't have planned a two shot suicide doesn't alter what it had become.
It may also be worth noting that recently, although Vanezis originally said she could have moved after the first shot but would have needed to hold her head up, he now says, categorically, that she didn't move. I find it difficult to believe that with a broken neck and needing to hold her head in position, she could have lifted and manipulated the rifle into the position required for the second shot.