Bill......to answer your questions:
1.) I did a real quick search:
340 Total Leipzig Games
35 were 15 Minute Scenarios
M15-076. First Day of Leipzig was played 4 times to be the most popular, so just shy of making the list. 8 were scenarios related to the First Day of Leipzig. 6 were bonus scenarios (4 of the Berlin Ops and 2 of M15-Bonus 10. Danzig: Assault on the Hills). 4 were Confederation of the Rhine versus Austria/Prussia.
The Blitz has recorded (Chad can correct me if I got the data wrong):
167 Total Leipzig Games
31 were 15 Minute Scenarios
15 Scenarios played were - M15-064. Action at Göhrde: Pécheux Reinforced
9 Scenarios played were - M15-076. First Day of Leipzig
So First Day of Leipzig seems to be popular.

2.) I too thought the different national matchups were a lot of fun! Played them solo mostly, but hope to play against someone when my time allows. But the idea is a good one and worked well given all the "nations" at Leipzig.
3.) It actually does get played, about half a dozen games give or take in recent years. I too had thought early on that there was too much Guard & your thoughts on it are quite right. Plus they were in their own lands now fighting. But the Allies aren't a force to just walk all over either. Results wise, things are actually split right down the middle with equal number of victories for both sides of the Club. With La Patrie en Danger and Napoleon's First Lunge North/South being the heavily played ones. I don't think it quite caught on with the French side of the Club when it was released, but it has gained some steam since. The massive maps for many of the scenarios are filled with a lot of options for play.
4.) I know, right! They still do....

13 games in the past two years and 142 since 2010. Those are some features that were nice, but I agree that not having so many files per turn is nice too. NIR is one that I probably played the most of too! I have gone through at least 3 CD's of it and made backup copies too. Played PTW a lot too. But something about NIR was beautiful, the mass of troops was splendid. Kutusov Turns to Fight was always a good one and since 2010 (when DoR came online) it is the most played scenario of the 3 games, at 25 times. 14. The Battle of Waterloo at 20 games with 03. The Battle of Quatre Bras and 03. General d'Erlon's Assault the next two. They have a following still with the two "notorious" ones themselves Richard Bradshaw and Mark Hornsby carrying their fair share.

I'll send you the sheet that will be helpful for what you're looking for about the Marengo scenarios. I'll attach it to the reply for your recent email. We were away for a week dog sitting at a friends and I am not a fan typing email replies on the wife's MacBook laptop, so have been catching up on correspondence.