2) The lower level Muertos Guards (lvl 11, passive) and Muertos Scouts (lvl 12, passive) can be found in Mirmir's Forest, at the location marked with a pin on your map, and they all look pretty much the same. When you have some of the feathers return to Trevor.
Further towards the east, between Nornil's Cave and Soul Harbor, you will find Muertos Warriors (lvl 14) and Muertos Lieutenants (lvl 17). But since the Muertos Warriors are highly aggressive, this is not a suitable hunting spot for a lvl 11 character.
3) Trevor, who earlier said people would pay a steep price and he would personally pay well, is a bit of a liar. You get 45 adena per feather no matter how few or many you bring back.
If you are already lvl 15 you can do the quest "The Few, the Proud, the Brave" for an additional income.