1) Yes. For Requirement #2 instead of "sound card on" you should just use if the PC is running the software, because that means windows has already loaded the sound drivers.
2) It doesn't generate much heat at all. Make sure you cover the exposed pins with something so you don't short things out, and or secure it well so that it cannot move and short.
3) The FB uses about 20mA to 80mA at 3.3v, or about a 1/4 Watt full boar. Each output can sink 500mA maximum. Recommendation of 300mA to keep things nice and cool in your enclosed space. 300mA is still way more than any of my relays use. Note that the FB sinks current, not source.
4) It will always be on as long as it is powered. Your choice for what you want. If you want everything off when the PC is off, then might as well wire it to the PC. Otherwise, it doesn't matter.
5) If you are using the voltage divider/isolator board, then you do not need extra resistors. Alternatively, you could just use a bunch of resistors instead of that board. You may want some general wires, or extra wires just in case something gets cut too short or who knows what. I hate it when I am in the middle of something and am a couple wires short so I have to go to a store. Usually it is at 3am so I have to buy some crap at walmart and hack it. Waiting is not an option.
6) If using FuseGL, you can do whatever you want. If using RRFusion, I have no idea. You may want to ping sonic on that question. I don't know how he has tied the actions together.