So, the receiver's bitcoin-qt wallet syncing the 'forked blockchain' through the sender's wifi? I'm really not sure, but I don't think that's really possible. But if it is indeed possible, I assume that it would be very difficult to pull off, and the sender probably needs to do some stuff to the receiver's wallet for this to work. But again, I'm not sure.
Very interesting question nonetheless. Let's wait for other replies.
it is possible
take normal internet. your fullnode can select any node. and have many connections to avoid the risk of a dodgy node sending a dodgy block(thats one of the points/security features). but if the internet is a closed internet where the only available nodes are that of a dodgy block maker. then yes the only block your node gets will be a dodgy block.
however. its complicated as the block will have to pass the difficulty threshold to pass one of the security checks. (blockhash needs to have certain amount of 0000's .. which means alot of hashpower to achieve such. meaning its expensive to achieve
so yes its possible. but not cheap.
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where it is definetly possible and practical to do. is the lite wallets that dont involve validating blocks at client/user level where the lite wallet only gets UTXO data which can easily be faked on a closed internet