1. Yes, only the cages are enchanted. A familiar may sneak around provided it remains undetected. However, most of the Sinless have sharp senses so they will interfere upon realizing that a familiar is there. They might either toss it inside a cage of appropriate size or dispel it away depending on the nature of the familiar.

2. The familiar may remain summoned upon capture, but it won't be able to use any of its abilities within a cage.