1. What exactly determines how likely it is for a creature to escape while being transferred from a cage (to pit or another cage)? In my recent fortress I have yet to experience a single orc or terrifying goblin escaping their handler, even when the handler gets spooked and spends 2 seasons wondering around the map dragging them behind (an entire orc squad and goblin ambush was basically being walked around the surface until I fixed the view to my pit).
I haven't had a problem throwing orcs into my archery pit either. It could be a relic from an earlier version that people assume still happens. I do cross train all of my dwarves to proficient wrestling, so that might factor in.
2. Can creatures break out of cages? I just had a terrifying goblin thief and a wrestler presumably break out of their cages sitting in my stockpile, as there is no way they walked to that area (cage traps and military on duty at entrance). I'm also missing the only goblin thief I've cage trapped, so I'm fairly certain he was the one I had in my stockpile, and its been a couple seasons since my last goblin ambush.
The only way I know how this could happen is fire snakes (this doomed one of my earlier forts). If your cages are wooden and near a magma pipe or pool, they'll spawn near by no matter what you do. I always import my magma from a safe distance via pumps and tunnels.