ipv4mapping may not be the problem - it may be kernel related

well, there was another possibility I hadn't considered, in that
the destination machine I am trying to reach is running an older kernel.

diffserv stuff that works on 3.13 on localhost and ::1...

   works only on ::1 on 3.9 and 2.6.35.

It doesn't mean that I don't have an error somewhere, but some
more kernel archeology is required.