I've no experience with crypto but a lot with UI. Is it possible that they were trying to send 500K for 1865 and entered the numbers in the wrong inputs ? Software asked them to confirm. They saw the correct digits and pressed ok.

IIRC, bitcoin transactions are structured to always send the entire wallet balance every time, and the way to avoid doing that is to send almost all of the money back to the current wallet address in a self loop. Then, any money that isn't listed with a destination becomes the miner fee.

So they may have just forgotten to add the field that sends the money in a self-loop, and by default the account was drained by the miner fee.

This is obviously a terrible design but it can't be changed.

it's done like that because it uses less bits in the payment script.

you can do it other ways, but it'll cost more. bitcoin's fairly freeform like that.

That's not how it works. You can't guarantee where the fee goes. It's assigned pretty much randomly to some miner or a pool.

Unless the transaction is only advertised to the miner they intend and they wait until that miner finds a block?