The killer is
Oblongamous
Since
"Oblongamous, was adjusting one of the snow sculptures that are dotted about the grounds" and is the Gardner, it will be easier for him to get an icicle or a sharp object made with ice, since it was snowing and also there are ice sculptures outside. Most probably, he should have taken it from one of the ice sculptures and may be that's why he was fixing one of the sculpture from which he took the sharp part.
Also
Oblongamous was working outside the folly with a snowshoes and was outside. Still there is no foot prints means he would have cleared the foot marks with a broom or something. Otherwise, there would be his foot mark.
So my assumption is
Gilles came to kill D'alar'cop from his back while he was writing, but when Gilles was about to stab him, Cop noticed him and blocked the knife with his hand which made the wound in his hand and blood on knife. In the fear that he will get caught, Gilles thew the knife there and tried to escape from the mansion. While he was running, Oblongamous caught him. Seeing Oblongamous , Gilles got some idea. He gave Oblongamous a handful of gold coins and asked him to kill Cop for him. Oblongamous agreed and put the coins in his pocket. He broke one of the ice sculptures and took a sharp piece from it. He entered the Lord's room and switched the light off so that even if he get caught, the Lord (Cop) won't identify him. He sneaked right behind Cop and stabbed him on his stomach with the icicle.