[Endoperez]

One answer to all questions would be "it depends". As Gandalf would say, there is no best or worse in Dominions, things just have pros and cons.

1) Small armies of powerful beings can often be faster. And especially with powerful summoned units, like Ether Warriors, they are very powerful and need some special tactics to be killed. Big armies can often withstand more losses, as they are expecting to lose units. However, some artillery spells (Fire from Afar, and later Flames from the Sky, Murdering Winter and some others like Leprosy) are deadly and work very well against big armies of weaklings. Weak units cannot really kill tough things on their own, but can protect mages who do the actual killing.
If you go for masses, many players summon Vinemen and Vine Ogres with either bonus items or Ivy Kings. Undead themes of Ermor get lots of weak undead for free, so you might want to try them to get the hang of it. Also, the base Ermor and Desert Tombs C'tis, and to lesser amount Carrion Woods Pangaea, use both undead and living units to get big armies fast. I'm rather fond of base Ermor units myself, although the mage/priests can do little more than reanimate.

2) Some people use their pretender as a conquering force that starts conquering province with the help of natural abilities and lots of magic. As an example, Ghost King (ethereal) can cast Quickness, Mirror Image, Stoneskin and Personal Luck after just Alteration 2 (turn 2 or 3) and be equipped with whatever your nation can forge with its income in those turns. Other spells like Mistform, Iron Skin, Fire Shield, Breath of Winter, Astral Shield and Astral Weapons can be cast with just few more levels of research. This can also be done with more combat-oriented pretenders, like Ulmish Prince of the Dead who can wear heavy armor without fatique penalty thanks to his encumberance of 0. And yet other pretenders conquer provinces after researhing some offensive spells, and others by being immortal and taking risks, like Phoenix (who is also good artillery mage).
Some pretenders are made to be good at site-searching. They can also often forge many different path boosting items (Crystal Coin, Skull of Fire, Moonvine Bracelet, Earth Boots) with which national mages can start to utilize the gem income. Rainbow mages die easily in battle, and out of it, because they rarely have hitpoints or any really useful protective abilities.
Most pretenders are built to fulfill one spesific role, and their actions are planned with their abilities in mind. As an example, if you have Son of Niefel with Water 9 to get quickness for sacred units, you want to plan his doing according to his powerful magic. As Breath of Winter becomes more powerful with more powerful Water mage, using him in battle seems like a good idea. Later he will probably combat through Falling Frost (area of effect and damage BOTH increased with high levels of water magic). He can also cast Murdering Winter if you have the gems. Because he fights in the beginning, I might get little Fire or Nature or even Astral to protect him (fire shield, regeneration, luck/astral shield), depending on my nation's magic. After Evocation 5 he switches to Falling Frost. If he is idle at some point and I have no other water mages, I'll cast Voice of Apsu or site-search the province he is in, to get some water income for future Murdering Winters.

3) I mainly use empowerment to achieve levels of magic that give me substantial benefit, quite often the ability to get other mages with that level of skill. As an example, Fire 3 mage can forge Flaming Helmet which gives +1 Fire fore 20 gems, lowered by forge bonus and even without it much cheaper than empowerment. Especially when I have boosters for the new path, empowerment to level 1 often gives me access to new spells. And if I have lots of gems of one type but no way to use them now and no real plans of how to get mages able to use them in the future, I might empower a mage to be able to use them, hopefully to get summon mages to use the gems.

3) Fast start is quite important, as the independent provinces are easier to conquer than computer ones, don't have to be guarded against indies and offer you more gold, resources and magic sites with which you can conquer more provinces faster. In multiplayer one seems to need supercombatants or SCs, and Bane Lords with just Wraith Sword and Boots of Quickness (3D and 2W needed, 30 gems total) can kill dozens or even hundreds of normal units as they get their life back and get rid of fatique faster than they receive damage. With just few more items and gems (Pendant of Luck) they become much more powerful.