It's not my intention, i just want to view different "angles" because it's a hard concept and i can't really imagina this "decentralized" money system, in reality, the first node would be the master node, also the "central" server, atleast for the next nodes.
The first node won't be a "master node", it will be the ONLY node.
The first node is only a "central server" for the second node, and it is only the "central server" until the second node has a full copy of the existing blockchain. Once it has the blockchain the second node can start mining, and if it does then the first node and the second node will be equal peers. Neither will be "master".
Every "full node" has its own copy of the blockchain.
There are no balances in the blockchain, ONLY transactions (and a header for each block).
You are also missing the purpose of mining.
A node that is not mining new blocks cannot modify anything in the blockchain, it can only receive new blocks (and transactions) from peers, check if those blocks (and transactions) are valid, add the valid blocks to its own copy of the blockchain (and share those blocks and transactions with other peers).
Each full node downloads and stores an entire copy of the blockchain from its peers.
If you create a new bitcoin-like currency, and you are the only one that knows about it, then you can start mining and create a blockchain. You can send transactions to yourself, and you can send transactions to burn-addresses. You can NOT send transactions to anybody else, since nobody else is using your system yet. If you don't like some of the blocks you have mined, you can start at the most recently mined block and delete blocks working backwards until you get to the most recent block that you want to keep. Then you can mine new replacement blocks and just never tell anybody about the previously mined blocks.
Once you have at least 1 peer, and you have shared your current copy of the blockchain with them it becomes more difficult to modify the history in the blockchain. This is because your peer will refuse to accept any modified blocks that you create unless/until you mine enough blocks to exceed the total proof-of-work that the peer already has.
If peers start mining, it will become MUCH more difficult for you to modify the history in the blockchain. The is because the blockchain that your peers already have will be growing while you are trying to modify your copy. You will be chasing a moving target when it comes to total proof-of-work of the blockchain.
If the sum of the hash power of all the mining peers exceeds your personal hash power, then it will become impossible for you to modify the history in the blockchain. The is because the blockchain that your peers already have will be growing faster than you can grow your copy. You will be chasing a moving target when it comes to total proof-of-work of the blockchain. And that target will be moving faster than you. You wion't be able to catch up.
Nodes that aren't mining will check to make sure that a transaction is valid before they relay it to any peers. They will also check to make sure that a block is valid and is built on top of the greatest proof-of-work chain before they add it to their own blockchain or relay it to any peers. As such, a node with a huge amount of hash power will not be able to force the network of nodes to accept invalid transactions or invalid blocks.
Modifying a transaction that is NOT yet in the blockchain would require you to get enough hash power to accept that replacement transaction such that the next block mined includes the replacement transaction instead of the original.
Modifying a transaction that IS in the blockchain would require that you to get enough hash power to replace (re-mine) EVERY block that has been created after that original transaction was included in the blockchain. You would have to exceed the total proof-of-work of the blockchain that everyone else is using, And if you are not the only miner, then you'd have to exceed that blockchain as it is still growing.
The number of nodes that have modified blockchains doesn't matter. Only the validity of that chain and its total proof-of-work.
In your example where you just "modify the same transaction in A and B, faking it", the C node would refuse to accept your modified blockchain, either because the block wouldn't be valid, or because your blockchain would have less proof-of-work than the blockchain that C already has.