The sub-2 thing was spectacular. Still, some folks got their panties in a bunch about it being just a spectacle and not an official race.
I think a head-to-head would be the same: it'd be pretty cool, but still you'd have some wet blankets whining that it's not a real race (even if it was).
The London Bekele vs Kipchoge dual should be cool.
On the Walmsley side-track: It was an attempt at 100k, but it was a hot day and it was clear he couldn't hold record pace for 100k so they said run hard up to 50 miles and set the 50 mile best time and we'll call it a day but you'll have to walk it in to the official finish of 100k for it to count. So you're both right.