The Fall of Voron

Question to the author: why make missions "impossible" at low levels? After capturing the third "village," there's no opportunity to gather troops, let alone figure out what's happening in the unexplored part of the map...

That's how it was designed ;)
Regarding heroes fleeing – this shouldn't happen. Perhaps you have a mod installed that enhances the AI, because the computer generally doesn't have the habit of fleeing.
As for Kurak – all recruited heroes will participate in the defense; you just need to wait until he gets closer to the Raven Peak. (It's not the most successful implementation, but I did my best).
Regarding the companions – I don't know, I probably messed up again. I think the price was slightly higher than the purchase price.