So...... after a long hiatus... I decided I would post. I finally opened my YF-19 recently (as I have been pretty much been in get and box away in storage mode for a while now), and I had the same difficult to separate neck section that a lot of people have been having. I managed to separate it apart after much needed force, but on the way to transforming it back... I could not for the life of me change it back to fighter mode. Too much force required, and I didn't want to risk it.
I think I figured out the problem, and I fixed it. Now mine transforms easily. However, I didn't use the sand down method like some people here used... in fact I didn't touch those square pegs at all.
It's kind of hard to explain.... so I will post pictures tonight of what I did after I get home.