if you give access to ETH before receiving the BTC, the user can scam you,
To avoid this, an intermediary program is needed, which will unfreeze access to ETH only after BTC is received.The issue is that neither chain's contracts can access the other chain's contracts. So any system that requires 'monitoring the other chain' won't work. Hence the commitment-style implementation that Submarine Swaps use is so far the best idea that I came across for swaps across chains.
The ETH smart contract cannot observe the BTC blockchain.
I understand. Thank you for reminding me of this. 
In my idea, networks BTC and ETH do not intersect in any way. Iintermediary program, which, just does not allow one of the parties (mixer and user) to scam, connects ETH and BTC networks (conditionally), checks the receipt of the required amount to the addresses, controls the fulfillment of the conditions. Otherwise who is running that program? How could one of the parties trust that the other party is running it correctly?
I would gladly answer if I knew the answers to your questions and had the technical experience to do so. I proposed the idea in general terms without having any idea how it could be implemented. Perhaps experts in the field could answer or have already answered, as in the case of submarine swap.I think your mixer idea just boils down to doing a submarine swap.
Maybe have a look into those.
There's also a video presentation by Alex Bosworth. Not sure if it's good, though.
It is possible and if so, then I apologize that I wasted your time with "inventing the wheel". Maybe have a look into those.
There's also a video presentation by Alex Bosworth. Not sure if it's good, though.
I don't know anything about submarine swap or and first saw the term in your post, but I'll look through your links and try to learn more about it. [/quote]
The discussion really turned out to be interesting and I learned about such curious things as submarine swap. I am also increasingly inclined to think that mixing services will need to change, become more complex and evolve over time in order to be able to provide their customers with the best possible privacy. I think after a while we will be able to see and try mixing services v.2.0.