Whenever a creature you can see that threatens you makes an attack against one of your allies, as an immediate action, you can compel that creature to attack you instead. When you compel a creature to attack you, you must first overcome that creature’s spell resistance, and the creature can attempt a Will saving throw to ignore the compulsion. A summoner casting this spell can choose his eidolon as the target of the spell.
(emphasis mine)
Since the summoner can already cast personal spells on his eidolon:
The summoner may cast a spell with a target of “you” on his eidolon (as a spell with a range of touch) instead of on himself. A summoner may cast spells on his eidolon even if the spells normally do not affect creatures of the eidolon’s type (outsider). Spells cast in this way must come from the summoner spell list.
So, the bold text above must call out summoners for some other reason. How does the spell's power allowing a summoner "choosing his eidolon as the target" differ from the summoner simply using Share Spells to cast the spell on his eidolon?
1. Is it because the range is no longer touch? Can the summoner thus cast the spell and simply compel the target to attack his eidolon? (If so, I would guess the target would have to be threatening them both.)
2. Does it simply mean that the summoner, once he has cast the spell (by touch) on his eidolon is the one who then uses his immediate actions to compel hostility, but that the eidolon is the one attacked?
3. If #2 is correct, does that mean that he can use Share Spells rather than the spells power of choosing his eidolon, and then the eidolon uses its immediate actions?
I want to make sure that I use this spell correctly (or as close to correctly as can be estimated without input from Paizo).
Thanks!
As I read it:
1) Yes. Yes. RAI I think the victim would only need to be threatening the eidolon.
2) No, if the summoner uses the eidolon as the target of a personal spell, whether by Share Spells or by this special clause, it acts in all ways as if the eidolon had cast it itself.
3) Moot since #2 is a No.