Below  is a recipe for quince preserve that comes from So Easy To Preserve book from the University of Georgia.  It is published by the university that administers the National Center for Home Food Preservation.   It is like the consistency of jam but they call it a preserve.  Quince is very high in acid so it is not a safety issue.  There is also a quince preserve recipe in the Ball Blue Book as well as in the Ball Complete Book of Home Food Preservation.  

Not sure the consistency you would get using the quince jelly recipe and turning it into jam.  

You could also use the Pear Preserve recipe and substitute quince and not have a safety issue. They have added lemon juice to their preserve recipe which is not necessary with quince.  

It shouldn't be a safety issue using blue elderberry juice in place of water if you are using the wild elderberries, not the American or European varieties since they are borderline low aciidl  With elderberries you need to use high sugar preserve recipes not low sugar.  I would suggest making a small batch to see if you like it.  Could have an interesting color and flavor. 

Hope that helps

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Nellie  Oehler