Some thoughts on MP.
1. The majority of RTS war gamers often veer away from MP because MP is where control can only be won or lost in the game, not outside it. People who play these grand strategy games love control. They are after all playing a game where you are in complete control of the faculties of an entire nation. Now you're telling them they have to play on speed 3, they have to adapt, they cannot save scum or utilize exploits. Most cannot abide by these simple expectations.
2. Given that there are no take backs in a MP game, I've seen literally dozens of people rage quite after a mistake. Not a mistake to lose the war, but just a major mistake. I cannot tell you the number of UK players who rage quit when they invade Northern Germany, which I purposely leave open, and then I swallow and encircle them with fast moving Panzer and motorized and they lose 20 divisions to the Reich. Or a Japanese player who quits after losing in China.
There will be some truth in your remarks, although it's worth noting that some RTS games have strong MP communities (Starcraft, for example). I've also noted that even in MP, people want control, but because of the fixed speed, the game can become a contest of APM rather than strategy (ie, the contest is decided by who has better mouse dexterity rather than who can plan and strategise better).
I also think the "most cannot abide by these simple expectations" is a bit loaded. I don't play MP any more, but have played plenty of MP in the past, but never in Paradox games for the same reason I don't tabletop wargame any more - the games were too long and too hard to coordinate for my particular lifestyle, even when I could keep up with things. I think it's awesome for people that do have the time/capacity to get those types of games happening (if I'm healthier when I retire, I'll lkely get into some GSG-style MP then), but these are very real barriers to a large number of gamers, and I suspect the main reason why long-form strategy games will always be primarily single-player affairs (but it's great that people with more time/flexibility are able to play MP as well

).
Another thing to keep in mind is that the lack of control of a major at speed #3 is highly historically implausible. It's still fun, but at least for me, I found it a barrier to playing. That said, I know many MP games are played at more historically plausible speeds, so I wouldn't see this is a barrier to MP per se, just a barrier to a particular type of MP (speed HoI, if you will

).
On #2, more people play ironman than MP, so I'm not sure if this is necessarily a huge factor in terms of the popularity of MP (ie, more people play SP where they can't take back, than play MP in total). Of course, people ragequit in MP, but ragequits are a pretty common gaming thing in any form.
something I am not sure if I'v said is that we removed the restriction on the difficulty sliders for not applying to humans, so if you play with people of very different skill levels you can use the sliders to give yourself some handicap.
I don't think you've said this, and this is really cool

. I don't play MP m'self, but this kind of functionality is a great way of a group being able to bring in new people without having to put "kid gloves" on, love your work

.