How (or if?) to isolate inputs to op amp summer?
I guess that one solution would be to buffer each of the inputs, but is there a simpler way?


Not really. You'd have to buffer them somehow, regardless of complexity. And op amp followers are of low external complexity.


  But that seems like it can't be right if what's behind the equal-value resistors is more resistors. What am I missing here?


You're missing buffering. The buffers would have high input impedance and low output impedance, leading to minimal losses on the summer inputs.