1.
Hunters are the main unit for clearing all mines. If your hero is a Warlock, you can get almost all rare resources by the first week... otherwise, you can try to do it by the second week, and if things are really tough, by the third week, you should have everything. Teach magic and have mana. I don't think Hunters are very good for the late game; it's better to make them into Assassins for the final battle.
2.
Megeras or Furies... it probably depends on your preference, but if you have rings that boost initiative, I take Furies, plus mass acceleration from attack – this is for the late game. For development, it's better to take Megeras for survivability, but don't use them in battles against mages, archers, and fast tanks... against zombies, armored units, and the like, they'll be fine.
3.
There can be a problem with Minotaurs: whether to build them or not. If you're short on resources, it's better to build Riders instead. If everything is okay, build Minotaurs, but don't take them with you and don't upgrade them. Then you just have to pray that there isn't a plague week... otherwise, the alternative is the second upgrade with 40 hit points and an aura of courage; they can hold their ground well. If your hero is a Kifran, you can even take the first upgrade; two attacks will be a bonus with increased stats.
4.
Dark Knights hit better with a charge... especially against those with high defense. They fight against armored units and Ents...
5.
My favorite unit for the Dark Elves is the Cave Hydra, and I like its appearance more, plus the abilities are good. I always take them.
6.
I agree that an extra archer wouldn't hurt. And the Necromancer is not bad either... for melee combat, Night Lords are too weak compared to other level 6 units; they can only cast spells.
7.
Yes! The strongest level 7 creatures in the game, and you can choose between magic resistance and destruction (damage is multiplied by 1.5) – awesome battles!
