Thank you very much for you patience so far. 
This makes no sense. The genesis node IS a checkpoint.
How do you verify a checkpoint? Either you do not (so you trust the checkpoint creator) or you verify it in some way.
So, new nodes have to verify the last block some way or another by e.g. asking their neighbors, watching TV, asking their local government, friends etc. etc.
I think we still mix up two different scenarios. This question is also related to NEW NODES:
Our merchant runs an EXISTING NODE here he should wait the number of blocks that satisfy his need of security as the blockchain approach leads to eventual consistency.
How does a node verify the genesis block?
The genesis node is hardcoded in the client.This makes no sense. The genesis node IS a checkpoint.
How do you verify a checkpoint? Either you do not (so you trust the checkpoint creator) or you verify it in some way.
So, new nodes have to verify the last block some way or another by e.g. asking their neighbors, watching TV, asking their local government, friends etc. etc.
I think we still mix up two different scenarios. This question is also related to NEW NODES:
Quote
Given two chains A & B which are different but equally valid without relying on a trusted third party tell me which one is the "best" chain?
This is solved by verification, code review etc. etc.Our merchant runs an EXISTING NODE here he should wait the number of blocks that satisfy his need of security as the blockchain approach leads to eventual consistency.