1) even though the diplomacy AI is on the same general competence level (meaning moderate) in both games, the AI tends to be much more aggressive in LH. It will solely look at your military rating and if you are beneath them they will hate you and very likely declare war on you.
2) each additional hero will half the xp for the entire army, so regular troops are also affected by it. The only real difference between sovereign and heroes is that heroes get an injury if they are defeated in combat and that you lose half your current mana if your sovereign is defeated in combat. Indeed, if you get lots of fame quickly you might have problems finding a suitable occupation for your heroes, but they can be reasonable scouts at the beginning of the game and collect all the items in goodie huts. You can also steal their spirit with a spell if you don't like their stats and if they have a magic school you want for your sovereign. Later on you should have enough money to support several armies for all of them, but there will always be a couple that sit in a city and decrease unrest.
3) drag and drop the icon of the improvement to the position you want it in the building queue when you have a city selected.
4) They can be quite useful at the very beginning of the game, because they are faster than other units and even heroes since they can move through forest, swamp, river and hill tiles without losing an extra movement point. Identifying valuable city spots is very important in this game. Because of this, they are useless for factions that have the Master Scouts trait, like Tarth.
5) If the value is ok for you and the faction you are trading with is not much more powerful than you, you should do it. The difference compared to Civ 5 is that you don't get a new tech for sure, but that the research time will decrease for a random tech within the specific research tree you are trading.
