There is an inactivity leak that gets triggered if you prevent with over 1/3 of your votes from the network reaching consensus. Inactive validators start to leak ETH from their stake until they become less than 1/3 of the total stake and cannot prevent consensus again.
There is no difference as far as I know. The network is supposed to finalize blocks that are almost 13 minutes old. If it's not able to do that because there is not sufficient quorum then it starts the inactivity leak phase and the penalties associated.
By the way, there is a bug bounty program. You can earn up to 1M USD per critical bug either in the protocol or its implementations.
Yes, but what is your point? You will be slashed no matter how hard you try to avoid it, unless you have 2/3 of the validators. The closer you are to 2/3, the faster you will be slashed. The system is self-estabilizing.
