... plus a few DSD files which cannot be streamed via the Squeezebox/transporter.
I think there is a plug-in that supports DSD - I don't know much about this area, so onward...
Questions:
1. How can I make the Main Library find the music files that I already have in the Drobo? (I take it that I do not have to move my existing files; just make MC look for the files already in the NAS).
Configure Auto-Import (or import a single folder) to point to the music folder on the NAS.
2. Once that is done, is there a way that I can use MC interface (instead of LMS) to send the files to my squeezebox/transporter?
Yes, it will be via DLNA, and you will not have gapless playback. The SBs should appear as zones once you enable Media Network (Tools > Options > Media Network). Also, you'll probably want to disable conversion under Audio under Add or configure DLNA servers....
3. What else do I need to do to send the DSD files to my DSD player that already shows in MC (Oppo 105)?
If you are sending via DLNA, MC will have to convert the format and stream as an SB/DLNA supported format. I think this will be PCM or FLAC.
4. Are there any settings that I need to use in MC in order to get the best quality sound?
Certainly you don't want to transcode your FLACs to MP3, so disable conversion (see above).