Welcome

1. As a new player you are facing the typical difficulties of not really being able to participate in everything. For the most part, try to get stronger as this game requires time, a lot, and the longer you play the more options aswell as power you get automatically. Being able to do expert is a start. Ultimate is more for the people that play a bit longer (a month or 2 can be expected to reach a level to consistently grinding those).
The difficulty of the Advent depends on the Advent itself. SOme are easier, others are harder.

2. You probably got 1 Phoenix dropped from a platinum chest. This can happen and leaves you with 1 extra copy. Just sell it to the Eidolon Shop for some Eidolon Orbs.

3. The only way is to check their member list of unions that either scout you or you look on the list. If there are a lot of members not being active for, lets say, a week, better stay away from them (no offense, but that is kinda not active in my book). Other then that, try to rank up, do as much as you can with your resources and try to get something good from the Gacha to build a team.

Many people can help you in the pinned thread for questions & answers aswell as there are resources found in some of the peoples signatures here (sanahtlig for example).