Can I suggest you read the primer first then come back with the questions. You might find a number of answers in there first. If it were possible to “buy off” nodes then the netowrk is insecure and we go home. So this issue is at the core of the network design and its best to read further so that you can find answers or expose the flaws better
No its a big problem for files/data. The coins are only data so if it can be done for data then it can be done for coins. Also many secure databases will be on the network and to be able to break into them would be worth more than a number of coins help by a section/group.