Questions about the gameplay of Heroes 6.

Yes, I thought so too.
And if you don't attack the opponent for a long time, their troops increase by 25% each week, and the same applies to neutral factions, right?

Troops increase for everyone; the percentage of their increase depends on the selected difficulty level (as far as I remember). Therefore, you need to act according to each specific situation: sometimes attack immediately and aggressively, and sometimes wait, first capturing nearby neutral forts and castles, thereby further increasing your army. However, delaying the attack for too long is also not a good idea – the opponent is also not idle and will also capture nearby neutral forts and castles.