Ah! I've just completed it. I was actually disappointingly close to victory in the video without realizing it - all you have to do from there is do a better job cutting the arteries, then lift the old heart out, drop in the new one and that counts as completion. I was expecting to have to jigsaw everything back together once I'd done that, so the perceived insurmountability of the challenge is... not as dramatic as it could be - but bearing in mind the game was made during a Global Game Jam, it's not bad, is it!