To answer your 1st question, you won't have any SQ advantage by choosing an iMac over a mac mini. I have both, and both use the same optical out that causes some to complain about jitter.
I agree, however, with the post that questions your setup at such great lengths. I had this same problem, which is why I eventually settled on buying an older, used mac mini and running it "headless" - i.e., no monitor. Apple has an easy to use screen sharing mode where you can access one computer with another Mac.
If you don't want to go the 2 computer route, then you really should consider using an apple tv or airport express to stream wirelessly. Both of these devices have a digital that bypasses the internal conversion and lets you pass a bit perfect signal to a stand alone DAC. I can tell you from experience that they work great and, assuming you're running a wireless N network with the router positioned within a reasonable distance, you won't have any problems with drop outs. The only downside is you're limited to 16/44, meaning if you want to go high res everything you play will be automically downconverted. There is workaround to this, but it requires that you have an iPad (any generation)and still limits you to 24/96: http://www.benchmarkmedia.com/discuss/feedback/newsletter/2011/06/1/ipad-streams-high-resolution-audio-dac1