Why are the NPC summons for Dragonslayer Armour not appearing?
Solution 1:
There are only two summons available for this fight.
Eygon of Carin can be summoned, provided you haven't killed him, or the maiden. His Summon Sign can be found in the building to the right of the stairway leading to the boss fog on the right side just beyond the doorway.
The other Summon Sign belongs to Sirris, who, as you mentioned, has a backstory leading up to this event. You will need to have followed her questline up to defeating Creighton the Wanderer, for her sign to appear.