Bah, I actually enjoy talking about dom3, probably just as much as Kebooo likes to talk about EOFS. But it's a good idea to post questions here.
Just an extra info on few matters:
2)keep in mind that there are three kinds of leadership - normal, which increases with experience; magic which increases with magical paths your commander has or gets(e.g.empowerment), undead leadership - which increases a lot with each death magic path, and less so with each blood magic path. These are all separate stats, so a leader with 80normal, 40magic and 10undead leadership abilities can lead up to 130 units.
There do exist magic items which can raise each of the three stats.
Magic and undead units left without their respectable leaders dissolve instead of routing.
3)on top of what EuchreJack said, let me point out that the battle AI does have a bit of free will, so if you'll for example script a spell that's not applicable(e.g.no enemy troops in range), the AI will choose what to cast instead. Also, if you script a powerful, gem-intensive spell, and the foe you're facing is weak(according to AI), it might decide to forgo casting that spell in favour of something weaker and thus preserving the gems.
Also, there are some mechanics that nobody really knows how they work. One example is exact mechanic of fear aura, and another one is the extra gem usage for making casting easier.
If you're interested in stuff absent from the manual, visit the wiki
http://dom3.servegame.com/wiki/Main_Page . The various FAQs are a good source of explanations for some of the more obscure stuff.