If you have both saurus guard and an astrolith bearer by a slann how do these abilities interact? The way I read it if there are any unsaved wounds on the slann before they are allocated you use selfless protectors and on a 2+ the wound is allocated to the Guard instead of the slann, and then you allocate the wound to the slann or the guards, depending on the previous result, and at that point that unit gets the 6+ save from the astrolith bearer (assuming everybody is within the correct distances). Is that correct?
It occurs during damage allocation. This happens after saves. So any damage you pass off will just directly stick to the unit. The Slann however would get the 6+ save vs damage from the Astrolith before having to roll 2+ to pass them off. That, or you would just roll for 6's on any damage that wasn't passed off for the Slann. And then roll 6's for the Saurus Guard. Either way, you don't get to try to save the damage twice.
You shouldn't be able to take the 6+ before the 2+, as selfless protectors happens before you allocate wounds and revivifying energies occurs when wounds are allocated. Or is there a FAQ somewhere that overrides those wordings?
There is an designers commentary about how to handle save after saves and passing wounds in the LoN section related to Necromancers. Pretty sure you make all saves. Allocate damage. Then the hero and the unit make their respective 6+ deathless minions saves.
You calculate the total amount of damage allocated to your Kloak/Slann, then pass them off on a 2+ and afterwards get to use any FNP saves (Kroak 4+, Astrolith 6+). I guess this is the FAQ LizardWizard refers to. The Ceaseless Vigil is the same kind of ability as Selfless Protectors.