Based on everything I've experienced with the Squad Battle's titles, massive scenarios just would not work in terms of solo play. The AI simply cannot handle itself adequately, even with expert scenario design, over the course of a long scenario. It struggles, oftentimes, with medium length scenarios. PBEM is a different situation, but even if you skipped the AI scripting, putting together a scenario like you're suggesting would take a someone a substantial amount of time just to input the units, then script them to enter a variety of time points. You'd have to find someone really, really dedicated to do this with an awful lot of time on their hands. I'm not going to speak for any of the designers I know, but I think most of them would prefer to craft a handful of really great small to medium sized scenarios than tackle something as enormous as what you'd like to see. Perhaps you should poke around at the scenario editors and try your hand at crafting something like this
Personally, I think the scale of the game itself is what dictates the types of scenarios that work the best. SB wasn't designed to be played on a operational type scale, that's for other titles to handle. To me moving hundreds of squads per turn in a massive scenario would be incredibly tedious and require way too much time, not to mention having to scroll up and down the map all day long to keep track of things. The game replay would take forever. Not my cup of tea, clearly.
Lastly, I hate to be the downer here (again), but any type of an alternate set-up situation similar to SP will never, ever happen with Squad Battles. It's really not even worth discussing, even in a hypothetical way because it isn't realistic. It's something that has been discussed on and off since I joined the SB community years ago and the answer is always the same.
Squad Battles has been around a good while now. Massive changes to the game engine aren't going to happen at this point in its development life. Instead, Tiller would simply create a new series, much like the
Panzer Battles is probably going be, although there hasn't been much public discussion about this new series in quite a while. Not sure where that stands.