Now you have a few choices to make that will affect several questlines and several quest rewards as well. This is complicated so to make it simple, the best options are to either Help Hornsent or Help Leda both times. This will give you all the rewards within 2 playthroughs and provide the best flow for the story. So, my recommendation is to either go Option 1 or go Option 2 with all following choices as that will maximize your rewards and lead to the best flow for the questlines but I will explain the combination of choices.
Option 1: Help Hornsent against Leda. Choosing this will do the following:
Give you Leda's Rune (obtainable elsewhere) and Ash of War: Swift Slash (Not obtainable otherwise)
Lock you out of Aiding or Invading Ansbach and getting those rewards later.
Allow you to summon Hornsent against Messmer
Possibly make a later battle easier or harder
Option 2: Help Leda against Hornsent. Choosing this will do the following:
Give you Hornsent's Armor Set and Falx(both obtainable later anyway) and Lacerating Crossed-Tree (Not obtainable otherwise)
Allow you to also Aid or Invade Ansbach and get those rewards later.
Prevent you from summoning Hornsent against Messmer
Prevent him from showing up later and make a later battle easier.
Option 3: Ignore the fight and continuing forward. Choosing this will do the following:
Allow you to Aid or Invade Ansbach and get those rewards later.
Allow you to summon Hornsent against Messmer
Possibly make a later battle easier or harder
If you choose this, skip to the fight with Messmer then follow "If Option 2" after you kill him.
If you choose Option 1 or 2:
If Option 2:
Option 1: Help Ansbach against Leda. Choosing this will do the following:
Give you Leda's Rune (not obtainable after this) and Ansbach's Longow(obtainable with Option 2 as well)
Allow you to summon him for 2 later battles and finish his questline to acquire his armor set(obtainable with Option 2 as well), the Obsidian Lamina and Furious Blade of Ansbach later(neither are available otherwise).
Option 2: Help Leda against Ansbach. Choosing this will do the following:
Give you Ansbach's Longbow and armor set (both obtainable with Option 1) as well as the Retaliatory Crossed-Tree (not obtainable otherwise)
Prevent you from summoning Ansbach in 2 fights later
Prevent you from completing his questline and obtaining those rewards.
Option 3: Ignore the fight. You don't get any rewards and may not be able to summon him later. Don't choose this.
Finishing the Shadow Keep
Now it's time for the boss fight against Messmer but you have a choice as you potentially have 2 Summon Signs but can only summon one of them for aid. My recommendation is Option 1. Option 1: Summon Hornsent on the right. Unavailable if you Invaded Hornsent with Leda. Choosing this will cause a later battle to be easier. Option 2: Summon Jolan on the left. Choosing this will cause a later battle to be harder. Option 3: Don't Summon. Choosing this will cause a later battle to be harder.