giuliomoro ok super interesting - I am able to get input plotting to the scope, it shows lots of nice activity when I touch the input pins as you suggested (this is more fun then I expected).
So I suppose the issue is probably with P2.30 DOUT as you said! That pin does look a bit busted, the continuity test works on it but maybe it's just not properly connected inside. also, it seems to be connected to pins 32 and 33 somehow, which feels odd to me.
Anyway, I think I'm ready to give up on this, but thanks so much for all of your help (@lokki too!). I've learned a lot in this process, and it's likely I'll be able to find something useful to do with this board.
one last thought - are the line out pins using the same connection as the headphone outputs? is there any possibility that using those would work instead? it would involve some work separating the boards out so I thought I'd ask first.