question about DAC's and HDMI

Yes, you want to go USB from the computer.  Sending audio through HDMI to the TV then have the TV pass that signal along to the DAC is unnecessary.

ditto

As for making sure it's using the DAC in the TV instead of computers, it does so automatically. When you plug in a device either USB or HDMI Windows treats it like a device ergo, a self autonomous device. Any audio (playback or recording) device which can connect through USB or HDMI has to have its own DAC. Ever seen those USB headsets? They have there own soundcard. This soundcard is also what limits the bit resolution and sample rate. So when you use one of those headsets you cannot use you better pci soundcard or whatever to up the sample rate or resolution. All information that passes to and from that device must go through that device's DAC. Similarly the DAC on your computer's soundcard can only effect the ports off it. It can't "backflow" over to another device. Now you could manually alter the settings via the Sound windows under control panel to use your computer's card, but that would make audio come from your computer analog ports, it wouldn't make the DAC on your computer translate the data and force it directly to the TVs speakers. Basically if you have it connected via USB your computer is going to use the DAC on the TV regardless and you don't have to worry about it using your computer's DAC ( I assume you want it to use the TVs DAC).

HDMI is essentially DVI + S/PDIF.  Passing S/PDIF through from the TV to your DAC should work just fine.  No guarantee that your DAC will process DTS/DD/AC3 or other encodings, and no guarantee that the TV will do direct pass-through though.  Going simply through USB will guarantee that you are not sending encoded signal, so no 5.1 encoding nightmares between TV and DAC.