Maybe I should try firing some additional players or try approaching from another side if there’s a detour?
I figured it out – I need to approach them with the condition that there are enough castles for Railag and Shadia, one castle each!
I had to fire one additional player – I gave him a minimal number of warriors and sent him to fight the gremlins that appeared this week, and I captured the white flag during the battle – then I could hire him in the tavern.
Railag was already level 30, and the skill-boosting trees of Silanna refused to give him a level-up, as did the stones, but the crystals of knowledge and magical rings on the map still worked. Fai’den is level 28, and Godric is level 27, Shadia is level 26 (all before the final battle). I only found two Silanna trees – one behind the “Northern...” castle, and the second – on the opposite side of the first, almost in the lower right corner, accessible through portals.