RME ADI-2 Pro and ADI-2/4 Pro SE both have 4 analog output channels.
They can be accessed via USB.

Have a look on manual page 40 (Pro) / 43 (Pro SE):

17.4.2 Class Compliant Multi-channel mode
… USB connected … will feed all outputs separately (8 channels).

The digital outputs can be addressed too:
AES, and Optical/Coax.

This can be useful for future extensions into a 6- or mor channels system.

Yes, volume control of channel 1/2 and 3/4 can be linked.

This is a Roon thing, so nothing speaks against that.
ADI-2 DA-converts whatever you throw at it.

All, up to 768 kHz.

There are DSP processing limits above 192 kHz (manual page 17):

”At sample rates 352.8 kHz and up the Bass, Treble and Loudness function is deactivated.”

”At sample rates 705.6 kHz and 768 kHz the number of available EQ channels is reduced to 2 (1 x stereo). EQ can still be used with Analog Input, Line Out 1/2 or Phones Out 3/4, but only one of these.”

Yes, including volume control which in this case is executed by the DAC chip.

”You won’t notice that, volume operation at the ADI-2/4 Pro SE is seamless and behaves identical in any mode.” (manual page 17 again).

If DSD Direct really makes sense for your case depends.

You sacrifice all DSP processings, including those inside Roon I think (speaker management).
Roon’s DSD to PCM conversion with a high sample rate is a valid alternative too.

In my experience everything from 88.2 kHz and above sounds the same.
For vinyl turntable I usually set 192 kHz, which is 100% transparent.