There is no limit to how far a node will reorg given a chain with a larger proof of work, as long as the chain also starts from the hardcoded genesis block (which means they cannot be completely “non-overlapping”).

As far as I can tell, the propagation of the new chain would go something like this*: The nodes first coming into contact with the “magic node” will start downloading and validating this new chain, until they validate far enough that its proof of work has exceeded their original chain’s POW. This might take minutes to days depending on the distribution of mining difficulty in this new chain, how many transactions are in each block (if any) and also on the node’s processing power and bandwith. At this point the node will start relaying this new chain to its peers and they would start validating it. Thus the new chain will slowly but surely propagate through the network until all nodes are synched.

* Based on some assumptions about how Bitcoin nodes should work.

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