1: Certainly a great chip for this solution, with some caveats later about memory/feature creep
2: The chip can be completely controlled "real time" via i2c or SPI. Those busses aren't generally available on a PC in a meaningful way, so if you want to use USB or RS232, you will need to put some chip in there to translate. Analog Devices use a Cyprus USB-i2c/SPI bus converter chip, in our own products, we use an Atmel or Freescale microcontroller with a USB interface. This allows us to do things like store a copy of the current settings in a more friendly manner on the target device so that you can plug the computer in, and it will pull the settings the DSP is currently using to populate the GUI so you can continue where you left off.
3: You will want to make your own GUI for your application. Sigma Studio would be a nightmare to support for most end user applications. Even if it were allowed by the EULA. We have done our GUI in C#, however, your choice of language for the GUI is pretty unlimited, as long as you can talk to the driver for your communications method.
4: Our OEM solution is 6in/6out with the ADAU1701 which, is probably about the limit for features vs channels for our market. To get this, we use 2 external ADC's and an external stereo DAC. This gives us 4, 12dB crossovers, 6 bands of eq, 5mS of delay, volume, phase, and a noisegate,limiter or compressor on each output channel. With a 2/4 or 6ch input, and a small mixer for 2 channels to sum inputs 1-4 into 5 and 6. If a full 6x6 mixer was needed, that would use more memory/mips which would have to be taken from those features. Going to 8x8 we would have to use a more capable chip. Your ADC/DAC combination may limit your options for sampling speed. Because of the clock options for our selected ADC, we can not use the 96kHz mode of the DSP. Which is ok, as we need the mips anyhow, but do pay attention to that if you need all of the clocking options, or go for a bigger chip with the sample rate converter.
5: Well, I think my answer to #4 answered that. I don't know what quantities you are looking for, but if they are high enough, there are some OEM bare board solutions out there you can just grab and use to implement your design with.
Richard