I'm planning ahead...
BobS has clearly stated that a new version of Sooloos will specifically run on a QNAP HS-251. I am assuming that I will be able to control such a system (to a MS200 end-point via iPad). Is this correct?
Yes, it's all Sooloos after all.
Roon is a monthly subscription service (if one chooses not to buy into a lifetime subscription), Sooloos isn't.
Just to be clear, Sooloos software is not purchased either, when you purchase the hardware you are just granted a licence to use the software ... you never own it.
With Roon, 'all that glisters isn't Gold' (Shakespeare). With all the good will in the world, it's a start-up company. I hope that it thrives. However, Meridian by comparison is firmly established.
Very wise to be cautious, but 'past performance is no guarantee of future performance' so it cuts both way really. The only certainties are death and taxes

So, without looking at Roon through rose-tinted spectacles, I'm simply asking if my premise of running Sooloos through a QNAP HS-251 to a MS200 is a viable alternative to Roon.
Yes it will be, once launched.
Personally I'd not worry about the hardware too much, going forwards there'll not be much in it, what's more important the user experience of using the two software applications ... And that of course is for the individual to decide on.