1) Basically impossible, because that would require getting control over the ship's systems. The Null can hack the god-phones within seconds, so it would take an incredibly skilled and fast hacker to take control of their systems for even a few moments. Once the Null know you're there, it becomes exponentially harder, since they would then attack physically as well as preventing hacking attempts. It's possible that a damaged ship could accidentally crash into another if its navigational systems were affected, however. If a character was well-versed in engineering and ship anatomy, they may be able to locate a navigational relay and damage it enough to manually override the controls and crash into another ship. This would only work once.

2) The bombs cannot be detonated remotely without access to the Null network. There is a delay of only a few seconds between pressing the 'detonate' switch and detonation. Their troops' bodies are essentially expendable, so they don't worry about losing one or two if a bomb must be detonated in 'person'.

Long story short: it's easy to get on board the ships, and easy enough to do some damage and steal things, and it's even possible to destroy a ship or two if you're quick and prepared for danger. Unfortunately, destroying more than a ship or two (once the Null know it's possible for people to do) or taking control of any of their systems without the assistance of something like [Name] will be far, far more difficult.