Yes I’ve triple checked everything in sight, I was working on this issue for an entire day and for the life of me I don’t understand it. There is CLEARLY only ONE vertex on the mesh but on the UV unwrap it shows TWO vertices with a gap inside the mesh - I checked normals, I checked duplicates, I made UV unwraps every time I made a modification - but I was still getting the gap - eventually I decided to delete the side of the mouth and rebuild it, and I still got the exact same problem! But strangely, I closed Blender and then opened it again, reloaded the file… and the UV mesh did not show a gap after that. However, it still had one side of the mouth much larger than the other, even after rebuilding one side of the mesh using the mirror modifier!
I eventually managed to solve that too, by applying the mirror modifier to create both sides of the mesh and then rebuilding both sides of the mouth by hand, making sure that I was mirroring what I did on both sides manually, which was obviously a pain to do without using the modifier again. So, now my UV unwrap is looking great and both the split-in-the-cheek error and the one-side-of-the-mouth-larger-than-the-other error are both gone - but I am now dying to know exactly what might have caused this, because if it’s happening to me now, its going to happen again in the future 
I spoke to a friend of mine who is much more experienced at Blender than I am, and he said that he has also had the exact same problems before, and he also had to solve it the same way I did. We are both wondering why this happens. If anyone can enlighten us, please let us know 
