Justification was there, just did a poor job showing it.
Would Ysh and the others come to fight Zooral Ja after the forces outside the dome were cleaned up? Yeah, the dragons have everything in hand by themselves, there was quite a few who showed up.
What went wrong then?
They showed up at the wrong time. They should've showed up before you went in the dungeon, giving you the option to use and of them, and those you didn't use will take an alternative route up so you're not placing all your eggs in one basket.
With the living memory? Graha going in absolutely made the most sense out of every decision they made in that scene. Ysh said it point blank, he's the only one with the experience of calling across reflections and summoning. This took him forever to research and get right, I wouldn't want to go to another hostile reflection without him.
What went wrong here then?
They made the same mistake as above. They told us but didn't show us at all. If characters are going to stay behind then they should have had two scenes:
1.) We fight as Thancred or Estinien to defend the casters keeping the portal open.
2.) Once secured we should have been able to play as most of them during the defense of the city.
That would've fixed it.
If there are instances where having 8 characters there doesn't make sense then just use Azem's crystal but have it use random player models and call them "Shade of ..." with a preference placed on pulling them from your FC / Friends list.