1) How strong are Humans? Can they compete with Necromancers and Orcs?

All races are strong in their own way. They can compete with both Necromancers and Orcs. But you need to assess the chances based on the situation.

2) What are the best and worst opponents for Humans?

It's hard to say, you need to look at the situation. Let's see who they might have problems with.
Inferno, it can be really difficult against this Castle. Knights have little mana, and if Imps steal it right away, it's very difficult for a Knight to stand against Darkness, and even an army of Demons with reinforcements. It becomes possible to take out the Paladins, and essentially there's no one left to fight Darkness. And removing enchantments from Adepts is not a guaranteed solution and may not help in battle. In RTA, this matchup was considered the most unequal, and Humans are given the biggest bonuses there, including 4 free knowledge points.
Mages, but only in the late game, with strong mini-units. Although it's difficult for almost everyone against such Mages.
Necromancers, but again, it depends on the situation. If there aren't a lot of undead units that can be converted from other Castles, then it's still possible to win, but if the Necromancer has a lot of Castles for this, then it's a problem, it's hard to fight this cheat. In general, this converter should have been limited somehow, as was done with retraining in Humans.
It will probably be easier against the other races, but again, it all depends on many conditions.
Against Elven Archers, it's good that Humans aren't particularly afraid of ranged attacks. There are armored units, and mass evasion can be countered.
They also fight well against Dark Elves, especially in the late game.
Against Orcs, there's Darkness in the home Castle, so it's also good.
It's roughly an even match against Dwarves.
In general, you can lose or win against any race, if you want to.

3) Do Humans need logistics? (considering "Huge" and "Enormous" maps)

Of course, it's needed for fast development. Unless you're playing a treaty game and not attacking each other until a certain time, then you can do without it. If there are many AI opponents, then logistics is also desirable, as you will often have to run after them.

4) Describe the tactics against Necromancers. (considering "Huge" and "Enormous" maps)

If the Necromancer has many Castles, then I don't even know how to fight them. Then it's better to eliminate them as soon as possible. Maybe Darkness with Sandro's Cloak will still work. Otherwise, it's desirable to have Light with Storm Wind - it will immediately reduce the Necromancer's 3 flying units, and also combat skills, more resistance and leadership with artifacts for morale, because if the Necromancer has a Creature Ability, then morale will be low.

5) Describe the tactics against Dwarves. (considering "Huge" and "Enormous" maps)

Train more Paladins, although you can also use Isabel's Knights, they might perform better. However, on such maps, you can expect a non-native school from the opponent, including Darkness from the Dwarf. And the development for Humans is almost always standard. In Light, you can take fire protection as a last resort.

6) Is it really possible to defeat a Necromancer on an "Enormous" map 1 on 1?

As I said, if the Necromancer has many Castles, it's too difficult, it's best to eliminate him as soon as possible, otherwise it will be extremely difficult to do. Unless you develop Darkness and look for Sandro's Cloak.

7) Is it worth developing Machine Management? (considering "Huge" and "Enormous" maps)

In the late game, they are of little use. And I rarely take them for Knights anyway, the damage of a Knight's Ballista is not very high, only for Vittorio, of course, I develop it, but otherwise it's better to take other skills. Although if there's a mentor, you can take it at the beginning to make the start easier, and then reset it.

8) Is it possible to rely on converting Monks into Knights, in a game against a Necromancer, and win? (considering "Huge" and "Enormous" maps)

You need to train additional Knights in most cases anyway, and I already wrote about the chances.

Don't underestimate Dungeon either. At least with Demons, Dungeon can compete on equal terms, and even have an advantage.

He said that as if Demons are weak in the late game. And I doubt they are the easiest opponents for the league.
However, you can also fight in the league in the late game if you get about 200% additional elemental damage, then Chaos and units will deal noticeable damage to opposite elements.