NDi audio is fine, there's plenty uses around, just avoid the integrated ndi in arena and use another solution to serve it. In fact, you can use the official ndi tools to send the arena composition with audio out of the box.
Combobox is even more complicated to me, and SLOW (is almost the same that pick up a deck from compositions panel, aside the modification-save thing). I don't want to type or check a list to pick one tab (usually named 01..36, misc13,various24..). I just want a click and see the next 30 tabs, or the...
I think DXV has no chance to screw up things on that side, is just AE.
Check if you're rendering with Premultiplied (matted) instead of straight alpha.
I bet you're premultiplying by lime, tasty but wrong for alpha creation.