1. yes. pg. 85 Tyranids rules specifically says that when an enemy gets gets into btb with the Tyranid ships. There is no specifcation as to who initiates it or at what point they come together.
2. Collapsed shields are still shields. when the shields suffer a crit, then they're in trouble
3. no. pg. 85 Tyranids rules state that no dice is rolled for holofields
4. Pg. 84 in both cases says that the 'Nid ship ship must inititiate contact.