Those are a lot of questions
The CM6610A seems like an interesting chip. I didn’t read the entire datasheet but it looks like there’s no mention anywhere of SCMS, so I wouldn’t be surprised if it will just decode the audio from SPDIF to USB and disregards the SCMS information completely. I can’t be sure though.
I think you may (also) want to look at the Teensy, particularly the Teensy 3.5 which I will be using for the DCC-i project, because it’s 5V tolerant. It can be programmed in the same way as an Arduino but it’s much more powerful. There is an Audio library available which makes it really easy to create audio projects. For example it should be relatively easy (a matter of drag-and-drop) to create a software project to read the audio from the I2S input, and let a Teensy stream it to a PC via USB. All you would need in theory) is an SPDIF receiver that outputs I2S to the I2S input on the Teensy. And you can use the other I2S port on the Teensy to stream the audio out again, e.g. to an SPDIF encoder. The outgoing audio wouldn’t have any SCMS restrictions; you’re the boss
I think you mean: Is the ITTS information available in the I2S stream that comes out of an SPDIF receiver? The answer is no. The ITTS information is transmitted in the User Data subchannel of the SPDIF signal, and it’s not part of the audio, so it doesn’t end up in the I2S stream. My SPDIF receiver project has some information on how the subchannels are encoded.
As you may know, I did some reverse-engineering on the ITTS box of the DCC museum. The page of that project also has a link to the datasheet of the Mitsubishi M51581 SPDIF receiver chip (whic, by the way, was also used in some recorders). I don’t know if that chip is still available but I imagine that other SPDIF receivers work in a similar way: They convert the incoming audio to I2S and have separate serial output pins with only the bit stream for the User Data subchannel and the Channel Status subchannel. The ITTS box that the museum has, uses one microcontroller to decode the User Data channel and one microcontroller with a Teletext video generator to put the information on the screen.
It shouldn’t be too hard to design a circuit with an SPDIF receiver like the M51581, that has separate pins for the subchannel data, and a microcontroller that reads that data and sends it to a PC via serial or USB. Then you can write a program on the PC that implements the ISO/IEC 61866 standard for ITTS. As you may already know, it’s very expensive to buy a copy of the standard, and the standard is very complicated. There’s no doubt that the producers of DCC cassettes didn’t use the entire standard as it exists now. The Teletext generator in the ITTS box of the museum can only do European characters, for example. But even if you would want to write a program that only implements enough of the standard to work with DCC, it’s going to take a while.