So what if the miner produces a pruning block for me nodes that has a higher block height than the actual? Then the miner expands on that block? Currently the best blockchain is chosen by having the greatest sum of difficulty, but this breaks that because it cannot be calculated with a partial blockchain. How would the node be able to know that the blockcthat it knows of is incorrect? If it receives the actual blockchain how will it know that is the correct one versus the incorrect one given by a malicious node/miner?
Very good points. As a core assumption of Bitcoin, a single miner can not replicate the difficulty of the entire network. Using that knowledge, here is a first-take at such a procedure to download and verify a pruned blockchain:
1) A new miner downloads all unique pruned blockchains visible to them. It is assumed that one of them will be correct because the Bitcoin network is the most powerful network in the world.
A sybil attack is still possible, you cant just assume it away.