Not talking about coin origin are we.
I suppose the point is to not have something seized from you which you do not want taken.
Anyway, these questions are now outside of/unrelated to common sense understanding of what a "finalized" payment is.
Definitely, but I don't think that mixers would be considered an acceptable "excuse" for dubious coin origin.
> The key holder has the ultimate power to simply throw away the key. There is no recourse for that.
True, but what is the utility of that?