In-universe, it's obviously Clara's arm
- The rings alone give it away; rings are a staple of Clara's costume, more so than probably any other character, and she's almost never seen without one or two of them on her right hand (and that's clearly a right hand)
- It's clearly coming from the "Gallifrey Falls"/"No More" painting, but the sleeve doesn't match any of the Gallifreyan wardrobes; the only other character wearing a black coat is Kate, but the composition of the larger scene means it can't possibly be her arm
- I don't see the hand as appearing significantly older than I'd expect Clara's to be; there may be some slight wrinkling around the knuckles, but then she is hauling herself out of a painting, i.e. putting pressure on her hand. Skin is squishy, and deforms slightly when you apply pressure to it
Out-of-universe, it's...almost certainly Jenna Coleman's arm
I've got no idea why you think she would need to contort her body in any unusual way to get into that position; it's clearly her right hand, and we can see the position of her right shoulder. It looks as though she has her shoulders square with the camera, her right arm reaching up to grasp the edge of the painting (presumably as leverage to haul herself out of it), and her head turned into the room.
This isn't a difficult position to get into (citation: I just tried it, using a door frame and an office chair), and certainly doesn't require contortionist training. In fact, here I am, miming the position in place (anonymized); it's slightly difficult without something to brace my hand against, but I don't need to turn my head at all:
If my shoulders aren't square in that picture, it's because I had to reach my left arm around to hit the "take picture" button on my laptop.
It's not impossible that they used a hand model to get a surreal-looking shot of Clara pulling herself out of the painting, but I don't see any obvious reason to suspect it's anything other than exactly what it looks like. If the shot looks odd for any reason, it's just perspective trickery.