1) Castings spells cause fatigue too, depending on the spell. Being armored while doing so cause even more.
2) You don't need to research blessings. Left to their own devices, your priests will probably cast them, but you canf make sure of that by giving them appropriate orders.
3) Yes, this is called Empowerment. It works for any commander, even a non mage. It costs lots of magic gems.
4) As far as I know, all spellcasters are commanders. Commanders meaning that they lead their own unit, even if they are the only one in it.
5)
Yes, unless you are waiting until you can afford a particular one (which shouldn't take that long) or until a particular hero shows up (which could take some time). I usually make a prophet of my best priest, as the higher level of holy are really powerfull. But a prophet will be at least level 3, so if you have only level 2 priest you can make anyone your prophet.
6) Resources are used to build units. They are available on a per province basis, and are not stored. Basically, it is a limit on how much you can build in each province
Supplies is used to feed armies. They govern how many troops you can keep in each province. If you don't have enough, the unsupplied troops start first to starve (moral penalty) then get diseases (very bad). Starvation can be cured simply by having enough food. Disease is more difficult
Upkeep is the cost (in gold) of paying for your standing armies. It is taken (at the start of the turn) from your treasury, so basically it is deduced from your income.
7) The scales give various bonuses : Order increases gold, productivity production (and gold a bit), heat and cold decrease gold and supply, Growth increases supply, gold (a bit) and province growth, luck gives you more good events, and magic helps research but reduces magic resistance.
Unrest cuts your province income and production. Population (with province type) determine how big they are in the first place. They are both quite important. 100% unrest means you cannot build any army, and your income will dwindle to nothing
8)The admin value of your castle, divided by two, is added as a percentage to your province income, so fortifications are good for income. They also bring a lot more ressources.
9) I am sure you will get better. Particularly if you haven't being using the scales yet

Try this : order +3, productivity +3, and fortified city. Take a armor heavy race (Ulm for example). Go crush everybody under the weight of that armor

With Easy AIs, that should work fairly well without having to do fancy things...