I've been playing this standalone mod for a few months now.
Lots of fun! It brought me back to Terraria. Showed it to some friends too. They liked it.
I've also experienced the vanishing npc problems. World events are disabled and I've played with or without zones, but they still (rarely) disappear. The only information I have which might help is that when I approached the old man, he vanished after opening an npc speech bubble and using an idle animation. I had to wait for him to respawn the next day before I could fight skeletron. No big deal, I was unprepared anyways!