1. you would need to control 51%+ of the hash rate in order to manipulate the network consensus.
2. you can put the entire history of the world as data and the hash would still still be 64 characters. this is because the hash is simply a representation of the data, so the data itself can be as complex or simple as you want.

Lightning:

1. the info on the side chains are not stores on-chain. only references to the side chain are stored on the blockchain.
2. information exchange on the lightning network is not the same mechanism as that on the bitcoin blockchain. on lightning, you can have countless exchanges and only the sum is settled, therefore, there is no scaling issue.