Firstly, I'm surprised at the inclusion of Donald Erb, who I have never seen as a very interesting composer.
Secondly, in the case of symphonists like Schuman and Piston, it seems difficult to pick one work at the expense of the others. Schuman's 8th is very fine, but does it really stand out so far from the other symphonies, for instance, the 6th? In the case of Piston, I can understand choosing the 6th (I have a copy of a rare LP of Munch conducting it), but one should not belittle the others. And I don't see "Mysterious Mountain" as particularly special amongst Hovhaness's huge output.
It might have been better to compile a list of ten unjustly neglected composers, rather than trying to fix on individual pieces.
Well said :) I totally agree with everything you have written.