In Dark Souls 2, what are the conditions required for a Way of Blue member to summon a Blue Sentinel?

Question

What conditions are required for a member of the Way of Blue to be able to summon a Blue Sentinel for help (via Covenant perk)? Does soul memory take part in this?

Additionally, will a help summon be triggered if:

1. the Way of Blue member summons an invasion via a Red Soapstone Summon Sign?
2. the Way of Blue member gets invaded by Bellkeepers?
3. even if the Way of Blue member already has one White Phantom (xor one Shade) summoned?

Answer 1

According to the Dark Souls 2 Wikidot Here.

For Way of Blue:

 You do not need to be human
 You do not need to have the Blue Seal ring equipped
 You cannot already have two co-op phantoms present

For Blue Sentinel:

 You must be human
 You must have the Guardian's Seal ring equipped
 You must be in an area where your covenant symbol is flashing white. You 
 will not necessarily be summoned to the area that you are in.

Soul memory does take effect when protecting Way of Blue members. I did read somewhere that it will not trigger if you summon someone for a duel, as this in not an invasion, it's a summoning. Unfortunatly I can not find the source for that at the moment. I will update if I do find it.