I think this thread got necro'd, but for if there's anyone else in that situation..
I similarly was terrified of being alone, particularly to sleep. For me, there wasn't much to be done except wait for me to grow out of it. No encouragement, reward, punishment or distraction would affect it, because that couldn't outweigh my pure terror.
When I got too big to sleep in my Dad's bed, he let me sleep on the floor in his room, as long as I started in my room. He also would sit with me for god only knows how long until I got to sleep. I wonder if this is why, when I got older and could sleep alone, I had to perform a weird specific ritual to sleep at my Mum's but not my Dad's.
What I'm trying to say is that don't assume kids to be playing up. They might be doing their best and really not wanting to disturb you.
And don't assume that it's solveable rather than manageable. Might need to find a "next best" solution, and just live with it until the problem sorts itself out.