Jim,
Yes indeed, the main advantage of this set up is the fact that valve setting will be easier. And it is a significant advantage because it's quite difficult to peer down between vertical faces to see where the valves are- even with the opposing cylinder stripped out. Having said that, some people are skilled enough to set valves without eyeball access, but that's far, far beyond my abilities.
It is quite feasible to have the valves sit inverted underneath the cylinders, and steam pressure does seal them without needing springs- a few thou float is all that's needed and the valves get pushed onto the portface as soon as steam is introduced. It's helpful to have valve rods supported at the front end in blind glands. The advantage as you say is ready access without dismantling smokebox etc. Once the valves are set however, access shouldn't be needed very often. Hopefully. And if it is, the smokebox can lift off fairly easily.
The original cylinder design did indeed have the two halves bolted together through a couple of lugs to form one fabrication, which then slotted into cutouts from above (or below), and was the main source of strength and alignment to the frames. Those properties are supplied by the substantial bufferbeams on this model so it's not essential the cylinders supply them, but it doesn't hurt to build in more strength if it's there for the taking I agree.
Having just said all that, please don't think I know what I'm talking about particularly. As you'll see as this loco progresses.
Jamie