Hi Brian,
1) I find the Core Control app great. Not as involving/immersive as C10/C15 but you can find/search for what you want pretty quickly or just 'swim' through your collection with ease... or search for new music using Rhapsody/TIDAL and add it to your collection.
2) I have used the RCA outputs from the MC200 for my STAX headphones and it sounds pretty good. If you are looking at getting some DSP5200 then I recommend a SpeakerLink pair.
These should have the latest EBA software giving better sound quality and using SpeakerLink (Ethernet cable) it's very simple regards to cabling (more convenient).
3) Easily backed-up to a PC or PC with external hard drive (my preferred method) using the Control:PC software.
4) I've not tried TIDAL yet as I'm waiting for MQA to be incorporated... then I'll give it a trial.
Even after listening to my main system, I'm still amazed at how good the MC200+5200 combination is. With the latest Sooloos updates on MC200 + EBA software on my speakers, its never sounded so good. You can always add an MS200/MS600/8X8/ to upgrade the sound quality if desired.
As others have said, there is a lot of uncertainty as to what will become of Sooloos/MDMS, how/if it's going to be developed. This is the gamble.
For me it's more about the quality of the music than the graphical interface (even though Sooloos has had a profound influence on how/what I listen to).
Even if Sooloos wasn't developed in the big step we've all been hoping for, for me it's a single box I leave switched on and lets me enjoy my music like no other system I've had in the past, without much hassle.
Meridian offer me a quality/Convenience that I'm happy to stick with... even happier with some MQA content to listen to.
I hope this helps a little.
Darren