I have yet to fully understand the routing within Orville and how to input a signal correctly.
I would recommend the Routing Utility, available free from the Web Site. This is really for the more recent machines, but if you set it up for H8000A and ignore channels 5-8 where dispolayed, you'll be close enough. I would STRONGLY advise close study of the factory routings as well.
let's say I have a mono synth
How to route this depends a bit on the preset and what you are trying to do, but if you route it to all 4 DSP inputs, it should work with most things.
You should not need Y cables, as you can do most things inside Orville. A stereo return to your mixer would be reasonable. Make sure you use a parallel routing if you are feeding A and B from separate places to mix their outputs.