About the missing chapter 11, after looking up for the collected volume it seems that the author messed up the numeration in the magazine and it got fixed when the volume 2 was compiled. Chapter 12 (magazine) is chapter 11 (volume) and so on until chapter 16.2 (translation) which is chapter 16 (volume):
japanese volume 2 => translation [and magazine, i assume]
10 => 10
11 => 12
not present => 11.5
12 => 13
13 => 14
14 => 15
15 => 16
16 => 16.5
17 => 17
18 => 18
3-page extra => not translated
So, no chapter is missing in volume 2. Iunno why 11.5 is not in the volume tho.