Thanks to everyone that helped me solve my namespace issues! Indeed, I broke
my posted testcase in the process of simplifying it. (I hate when I change
something from properly broken to improperly broken.)

With your guidance, I have it working now. I posted the solution on
stackoverflow.com to make it more findable:

	https://stackoverflow.com/questions/64716407/for-mixed-namespace-xml-content
-how-do-i-prevent-redundant-xmlns-definitions-o/64716408#64716408

All the best,

 - Chris