(1) Weapons differ in attack, defense, length, damage, number of attacks, special abilities (vs. shield bonus, for instance, armor piercing (ignores 50% PROT), etc). Some benefits are found only on magic weapons, such as being armor-negating (ignores PROT). Crossbows pierce armor. Electrical attacks negate it.
(2) Composition of enemy force esp. surviving mages, fliers, unusually tough units, magic items. Interaction between units. Tactical problems such as friendly fire, not having enough gems, etc. Bars indicate hp/fatigue damage. Icons shown are all units in square; 6 points of size fit in one square (hoburg=1, man=2, cavalry=3, jotun=4, titan=6).
(3) Orders used frequently include hold/attack if I want the enemy to have to come to me, fire/archers or fire/rear at times, attack rear for flankers esp. cav, fliers... Assassins, powerful mages tend to get scripted by me, as well as spells that I decide I definitely want.
(4) I don't, but it would make some sense to do so if you've the spare leaders.
(5) You can't.
(6) 1 man-sized unit per water point, IIRC. It won't let you take troops that'll drown immediately.
(7) Every defense point gets you more troops. At 20+, the troops you get per point change.
(8) Depends on overall strategy, pretender, nation, enemies, magical resources available, size of map, strength of indies... no right answer.
Site searching again depends, e.g. Ermorian Ashen Empire Lich Queen, which can have absolutely insane amounts of magic, frequently makes a good searcher. Divination spells can be very nice -- they're cheap, too, and can help a lot if you have poor access to level 3/4 mages of the appropriate types. Some require level 3, but such a mage might be more useful at home summoning, forging, and casting rather than searching.