Putting a DI between the computer headphone out and the mixer should
take care of noise introduced through that analog connecting.
I would go with a passive transformer based DI, the amount of true isolation
with an active DI can vary from DI to DI.
With most DI’s you would need to use the XLR inputs as the DI’s output closer to
a mic level signal and if you using an active DI you will need phantom power to
operate the DI, some will take a battery.
The ST1 and ST2 inputs are unbalanced, the channel line input jacks are balanced.