Zippy,
a bit to much for me to wade through point by point so I'll try to give you some general ideas that should help out in any situation:

Audio invariably starts out its life as a very small voltage, typically mere thousandths of a volt. At a S/N ratio of 60dB the noise is at a millionth of that. Clealry it doesn't take much to add noise to the audio from mics etc. Apart from having unwanted bits added to your sound bits maybe removed due to limitations in frequency response. Also some gear may introduce phase errors between different frequncies altering the sound space. Also the actual shape of the wavform maybe altered causing distortion, this maybe related to the nature of the signals themsleves.

Now that's just the analogue side of things. When those signals are converted to digital the quality relies largely upon how many bits are used to reperesent the voltage and how quickly they are sampled. Also the design of the A/D converters may introduce artifacts in much the same way as the analogue circitry does.

So what to conclude from all that and your original question?

I'd suggest that getting the low level analogue signal processing outside of the PCs case is a good thing. PC power supplies are very noisy and secondly it's never convenient getting to the connectors. That's not to say there aren't some excellent PCI sound cards around. Also there's limited space for connectors, a few XLR connectors would eat up a lot of real estate on the back of a PC.

Typically setups that have a separate box to handle the audio I/O have the A/D converters in the box but not always, you may need to read specific reviews to determine that.

Big difference from what I can see between usinf USB and firewire to connect external box is the available bandwidth. F/wire wins hands down.

I'd give the Soundblaster a wide berth, not really as good as it seems, particularly for recording.

The external mixer into a good sound card with real line ins should work well. Just be careful of the consummer sound cards, the one on my mobo seems to incorporate a lot of compression / limiting giving very poor real SN even on line inputs.

Hope this helps you at least get some insight.