It would be nice to have Output Channels and mixing channels as 2 separate choices. I think this would enable you to get rid of the (inside x channel container). You could then select 24 channels, but 5.1 mixing.
Instead of a drop down box, the mixing could be an entry box. This would let you put in whatever you need such as 2.0, 4.0, 5.1, 6.1, etc.
I really like the Audiolense method of specifying physical speakers on one page, input channels with another page, and output channels with a third page along with the ability to label everything to match your system. This lets you basically do the following:
Speakers: I have 5 speakers and 4 subwoofers
Input: I want everything mixed to 5.1
Output Channels: I have (1) Left bass, (2) left mid, (3) left tweeter, (4) center mid, (5) center tweeter, (6) right bass, (7) right mid, (
right tweeter, (9) rear left, (10) rear right, (11) surround left, (12) surround right, (13-16) subwoofers
For this one would want to be able to at least select 16 channels with 5.1 JRSS mixing in JRiver.