Tunesmith: while I disagree with most of what you are saying here, you've put your finger on one problem that certainly exists. The incompatibility of intonation thrown up by Vassar Clements and Stefan Grappelli trying to play together is always going to create problems. When players from radically different traditions attempt to collaboarte, there will always be a tension, unless they are both very empathetic and able to compromise hugely.(They tension may be intonation related, but it can equally well be rhythmic or tone-related). A compromise may not be a good thing anyqway, if it resuklts in one or both musicians throwing away part of what makes them musicians.
There will always be differences of intonation when a fixed pitch instrument plays with a variable pitch insrument, or when two variable pitch instruments meet. Whether this results in good or bad music is basically a matter of personal taste.