Besides, the Grail in the Tower or the Conflux is an unfair advantage. If it were possible to simply prohibit placing it in these two castles...

You could simply remove the map opening and all spells. Moreover, the Tower already has a bonus (+15 magic power), and you could introduce a bonus for the Conflux – for example, a resistance or magic bonus when defending the castle.

Regarding the "grind" stack – yesterday, I played with four friends in TCP/IP (and we play without online rules). In that game, "attacking with a hidden unit and retreating" made the game. In one turn, I reduced the number of Sea Serpents guarding the passage from 38 to 13 using six heroes. If I hadn't taken the first number, I wouldn't have been able to do it, but I managed the second almost without losses. And after that, all the good stuff in the center of the map (the M-castle) was mine. That's how it goes...