1) As far as I can tell, friends disappear from your friends list after you use them, and reappear after they login and do some activity. So if you have one friend in your entire list, it will disappear after you use him and you'll be forced to bring in non-friends. Leader auras do not work with non-friends so this will decrease your power significantly. Feel free to correct me if anyone has more specific details.
2) No, my water element team in particular is stronger than all my other teams, so I am seeking people with water element eidolon leaders to boost that team. This is because I drew into multiple water SRs and Shiva, AND I have a maxed out water element assault weapon from the past event. If you did not draw into a lot of good water element kamihimes then you probably should not build a water team or use Jorm/Garuda as your leader. The correct build for you is probably going to be different.