Restarted the mission. It seems to be going better than last time; I completed two quests – I captured a town starting with the letter E and killed the orange hero, the main demon, who challenged my Irina to a duel. I even reached the boss – the mother, the progenitor – but failed there, despite having a rather decent army. So, I decided to leave her alone for now. I defeated the demon thanks to the army I gathered on the island, where not only nagas joined me, but also snow maidens, shark guardians, and improved kappas. And, what's nice, this time the red and orange factions are fighting each other!

Added 38 minutes later
Interestingly, I just completed the "Fisherman's Village" guardian quest – I cleared the lighthouse, and, as before, I received a castle. However, surprisingly, my "Come in Peace" quest didn't fail this time, as it did last time. HOW STRANGE! I did the same thing, but the result was different.