How can I disable threading in some (e.g., mail-) groups, or set other variables specific for some groups?
While in group buffer move point over the group and hit ‘G c’, this opens a buffer where you can set options for the group. At the bottom of the buffer you'll find an item that allows you to set variables locally for the group. To disable threading enter gnus-show-threads as name of variable and nil as value. Hit button done at the top of the buffer when you're ready.