I don't think you're being a dick at all. And I really appreciate you taking the time to talk me through this and help me understand it. You're correct when you say I dont know much about what I'm trying to do. I have a computer science degree and the only audio knowledge I have is what i've learned installing the few car systems I've done and working with home audio equipment. My knowledge range on this subject is extremely lacking.
Where my little brain is getting hung up is the fact that I feel what I need to accomplish is sooo simple yet I can't figure it out. You're showing me that intercepting the audio will not work easily/cost efficiently. So maybe I need to approach this from a different angle. Maybe I just need to use the forbidden Y splitter backwards, but physically cut the connection to the headunit when I'm using the PC and vice versa when using the headunit. I'm not really sure yet. The last day or so my mind has been on how to split the audio signal. I have to sit down now and try to think of some other method that prevents signal outputting from the headunit and damaging the soundcard. I just can't believe its that difficult.
If there was a headunit that would input 4 channels as AUX (dont need left and right I guess, just highs, mids, bass, and sub) and pass them straight through to the amp (whether it be via RCAs to an external amp or through its own built in amp then straight to the speakers) without making adjustments to the processing...that'd be perfect. Or a digital input that would except any number of inputs then pass through without using its own processing. I'm going to go do some reading on headunits and see what i can find.