Nodes cannot be attacked to influence the network the same way as miners do. Nodes only enforces the rules locally and does not affect the protocol rules being enforced by others. Even if the other nodes accept a Bitcoin block with a million Bitcoins in block rewards, your node won't, as you're still enforcing the rules yourself, unaffected by what others are doing. As long as your own nodes is enforcing a set of rules, no one else can make your node violate those rules.

That being said, Sybil attack doesn't require any proportion of the nodes to be compromised and that is practically the only attack which uses the nodes. You're confused about 51% attack, I'd recommend you to do up more reading on this matter before posting any further.

Theoretically yes, practically no.

As there is no centralized server and the there are thousands of full nodes that store the entire blockchain, it's not possible to manipulate the bitcoin blockchain.
A hacker may access to the computer of someone running a full node and change the data. But it's not possible to take control of thousands of computers at a same time.

In theory, a hacker needs to have control of more than 51% of the hashing power for a long time to compromise the bitcoin blockchain. It's extremely unlikely.

 You can manipulate the data stored in your computer. But there are thousands of other full nodes that have the correct copy of the blockchain and reject invalid transactions/blocks.
A transaction needs to be validated by majority of the nodes.