Can bitcoin core verify a third party blockchain download? If you were to obtain a copy of the bitcoin blockchain, i.e. a copy of someone's data dir excluding his wallet files on a removable drive or other storage medium, could bitcoin core verify that or not? This would be useful in developing countries where the initial blockchain download is hard to do but keeping up with the chain is easy.
If you give Bitcoin Core a fully populated data directory, it will use it without any validation.

However, if you only give it the blocks/ subdirectory, it will fully validate it to recreate the chainstate directory, exactly as if it were received over the network.