Seeking Eidolon (Unchain) Experts for PFS Legal

[Le Enigmax]

I am finally starting out as a new summoner.
There is three questions I would like to settle quickly.

1. When an Eidolon is Slain, is it banished back into its planes at -1 HP or when it reaches sufficient damage for CON death...

2. With the above question, for LIFE LINK ability, do it activate when it reaches 1 HP or when it is 1 HP to CON death

3. Can an Eidolon take EXTRA TRAIT as a feat?

[Le Enigmax]

Hi There fellow pathfinders. Anyone with an answer to this?

[Rengarth]

You activate lifelink to prevent it from being banished.