How crowded are the lanes with many players?
With 11 players it will be somewhat crowded, of course. However, the more heroes there are in a zone, the easier it is to make a mistake. This will result in a more bloody (and as such - more fun) games.
How feasible is a 2v2 on the 2-lane map? It should be the same as 1v1 on the single-lane map.
I wouldn't recommend it. You want more player-to-player interaction. Playing a 2v2 on a large map results in limited interaction between players in different lanes. Plus, the lanes on a 2-lane map are one zone longer. So it's not exactly the same.
Would a (custom) 3-lane map (like computer MOBAs) make sense for, say, 11 players? For the 3-lane map, I can imagine that physical space may be too tight, because the areas need to have a certain minimum extent in order to work well (with quick travel and special abilities and spreading out minions).
The very first prototype of Guards had a huge 3-lane map which was dropped. It was scaled down to the 2-lane map and, eventually, the 1-lane map came into existence.
Personally, I believe that a solo lane map works best even for 6 players. I don't think that you really need a 3-lane map. Plus, it would require more minions.