How do we know that there are no blocks that predate the genesis block? A block that could potentially contain any number of coins. If somebody already created such a block from before the genesis but never published it could it (technically) stil be merged with the chain?
The design of the blockchain is such that such a scenario is not possible. By definition, the genesis block does not have an ancestor block. While it has a field for the previous block hash, this field is set to all 0's to indicate it is
null and has no value set. This means that there are no blocks that come before the genesis block.
Additionally the genesis block is special in that it is both treated specially and hard coded into software. Nodes do not download the genesis block - they have the block in its entirety written into the software itself. This gives the genesis block special treatment, with one such special treatment being that the view of the blockchain is instantiated with the genesis block as the very first block.
While it is most certainly possible (and probable) that blocks were created before the genesis block, these blocks would have been ones that Satoshi created during testing and belong to a different blockchain. Those would not be valid on the Bitcoin blockchain because they refer to their own genesis block, or are their own genesis block. The actual time they were created does not matter, only whether they refer to blocks which eventually refer to Bitcoin's genesis block.