I believe that MC's conclusion, that they should be save from further major harrassment or attacks by the Black Demon lord for the time being is absolutely unfounded. The author himself wanted to show, that MC has made a mistake by letting him be attacked by a lunatic Dark Demonlord Cultist and made him rethink the current situation. I have to give it to Author-san, that he wrote MC this way to show that MC is naive in such a way and that he highlighted it just a few panels later with that one attack. MC naivity still irks me though.
It's all part of the game. Attacking with his own army is a no go. Making other do it for him is fine, as we already saw. I'm sure he (should) know ways, so he has to risk nothing.
Nope. The thing is I believe, that the author will still let MC's plan work out (and give that plan to develop a city the plotarmor), eventhough there are a gillion ways for the demonlord (and others) to sabotage it. It happened so many times in other mangas, that friendship and honest, earnest ideals ruled supreme, even though there have been opponents that had all the means they needed to destroy them.
Don't get me wrong. I do love a happy end. If the good guys win, while holding up their ideals, thats a good thing. What I hate though are nasty asspulls, the bad guys not using obvious methods, by simply not noticing them or using a more stupid tactic. (If they disregard certain tactics, because the have a good reason (pride, dislike of taste for a method or greed, for example), that's fine.)
I fear, that a naive plan will succseed, because Black Demonlord holds the idiot ball for too long or other events will occure conviniently pulled out of some hole, which is not further disclosed in detail here.