fior question 1, I would think tghey would be identical, since you are using the same type of input, but I would choose D1 personally, just seems more base level to me, but yeah maybe brian or someone will chime in, I cant see there would be an advantage either way.

for the second question, you can use an OTTO to switch between them, I will be using an OTTO to switch between the entire output of the MUX and an I2s connection. all my places on the mux are used up, but I would think using the OTTO would be a better choice than adding a electro-mechanical switch. unfortunately, the OTTO is out of stock for now.

A few facts (I think):
If you want to use I2S and/or DSD the SPDIF switch has to be off.
If the SPDIF switch is off D1 will recognize SPDIF but ONLY if it is TTL level.

So to answer your questions:
1) No. They are not redundant. D1 requires TTL level SPDIF.
2) Yes. If you want to use an OTTO to switch between DSD and SPDIF the SPDIF has to be send to D1, it has to be TTL level, and you have to put the SPDIF switch in the off position.
You can get TTL level SPDIF from a TPA MUX.
I'm using an OTTO to switch between the SPDIF from a TPA MUX and a DSD source. You need to fiddle a little bit with the MUX to make it work, however.

Nic

Thanks, Nic and Brian. Now I have to consider tapping the signal inside the player to get TTL signal instead of getting it from the coax out.

Clarence