Inferno tactics.

It's interesting about diplomacy – I hadn't considered that strategy. It might not even appear as a skill for any of the heroes. Or is the idea to level up each purchased hero a little bit at the first opportunity so that at least one of them gets diplomacy?

Yeah, that's usually what I do. I prioritize the hero classes that have the highest chance of gaining diplomacy. At the beginning, you need to buy nine heroes anyway (8 are running around, one is in the Castle) for the quest chains, and there are usually 2-3 resource nodes near the city that give +1000 experience, plus at the beginning, you're fighting with whoever you have without a clearly defined main hero. In my experience, diplomacy appears in about 80% of cases.

For Inferno, having money is rare.
I haven't played in a while, but I recently tried playing on 200% difficulty against 3-4 AI opponents on M maps. On simple templates. It's quite difficult, I must say; usually, I can't develop well and have to admit defeat. Of the neutral Castles nearby that can be captured, there is 1 or 0 (depending on the template), so you can't really get a lot of demons that way.

Inferno usually has plenty of money, at least in my case. First, the Ifrits raid the Tombs, Dungeons, and Gnomeholds, then the Demons destroy the Utopias. I've never had a problem with money. I also haven't had any problems with getting enough Demons. Usually, there's one town-village on the neighboring territory, then an enemy Castle is captured by the Ifrits around day 126-127. Then, the caches of the first and third-tier units are usually scattered around the cities. In general, there's plenty of loot. Of course, there are sometimes very poor maps, but even then, you can at least get a hundred gold.
As for the AI... I haven't played against them in a while, to be honest. The last time was about two years ago when I defeated seven AI opponents in an alliance on standard M and L templates, and I got a bit bored with them. Now I mostly play with friends. By the way, I suggest we play a game if you're interested :)