Canon is what you see (or can see, depending on choices) in the game. Extra/Extended is a canon scene that then goes on to show more than you can see in the game, so it adds a fantasy element to the scene. You can't see these in game, so it's not a choice you've missed.
It's not Ashley, you talk to her and she goes into the room on the left and then you can open this door on the right and get the anal view, so it can't possibly be her. I don't think we see her elsewhere to check the nail colour, but it's likely Ivy again.
This is before Ashley would have the option to join or not join the sorority, so she wouldn't be given this role yet. If she does join, you later see her doing blowjobs as part of her early tasks. She wouldn't move on to being a full party prize/doing anal that quickly (no basis for saying this bit, it's just assumption).
Besides, that right hand has four blue nails, Ashley only had two of the nails on her right hand painted blue at this party (and it looks a lighter shade of blue):
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