There is a software called Youmix.
At a time, few mixers were using it IIRC, and I believe it's still used nowadays by 1 or 2.
Is the spelling correct? A Google search gives different results that are completely far from cryptocurrencies. Even adding the word cryptocurrencies gives the name of a mixer and not a program for creating a mixing service.
The spelling should be correct. But looking on https://bitcointalksearch.org/topic/bitcoin-mixer-white-label-what-happened-to-youmix-5358101, looks like it's no longer exist.
But just like using mixer, you could decide when to perform XMR to BTC conversion. If you don't mind higher fees, you can even convert XMR to BTC in few different transaction.
Yeah, but it's not mixer or exchange responsibility.