I just don't know which topic to assign my question to... Please tell me, how will it all end? What will happen to Railag and Izabel in the end? It's probably silly... but I'm interested in their love story.

And Railag isn't even in "Lords of the Realm." Since Tieru sent him on a business trip to "Lords of the North," that's the last time we saw him...
Izabel will kill Biara and renounce the throne in favor of Frida.
As for love, as far as I remember, in Heroes, there's always a slight hint of the possibility of incorporating and tracking a love theme in the game's storyline, but then there's a sharp departure from this theme (remember Catherine and Roland, Yog and Vidomina, Sandro and Vidomina). And that's understandable - after all, it's primarily a strategy game, not a soap opera.

Added after 19 minutes

If possible, could you briefly outline the plot of "Lords of the Realm," as I haven't gotten to it yet...

But why do you need it now? It won't be as interesting to play later...