That majority is not a large number, as it is a feature of the Hive blockchain to use a small number of witness to achieve it's four second transaction validation consensus.
Bitcoin, the largest chain of them all has (at time of writing) 16843 nodes, where Hive has 100 nodes and only the top 20 doing the active validation, with the remainder being backup. So it would not take much pressure to influence the network.
Cast your mind back to the issues when Justin Sun attempted to take over the chain. He was only exercising stake, and not using the intimidation of police violence, arrest, jail and asset confiscation, all of which the state has at it's disposal.
Yes absolutely. The issue with Bitcoin is getting all of the people to agree to change. As numbers increase, security increases but getting everyone to agree on software updates also increases in difficulty.