1) Can't say for sure, but I've been led to believe they use whichever has higher quality. I'd recommend taking the modding route, like so:
[ITEM_WEAPON:ITEM_WEAPON_BOW]
[NAME:bow and scimitar:bows and scimitars]
[SIZE:300]
[SKILL:SWORD]
[RANGED:BOW:ARROW]
[SHOOT_FORCE:1000]
[SHOOT_MAXVEL:1000]
[TWO_HANDED:0]
[MINIMUM_SIZE:15000]
[MATERIAL_SIZE:3]
[ATTACK:EDGE:20000:4000:slash:slashes:NO_SUB:1250]
[ATTACK:EDGE:50:2000:stab:stabs:NO_SUB:1000]
[ATTACK:BLUNT:20000:4000:slap:slaps:flat:1250]
[ATTACK:BLUNT:50:1000:strike:strikes:pommel:1000]
This will replace bows in your game. Alternatively, you might want to replace a weapon you never use, like Mauls or Morningstars. Just copy this into your item_weapons.txt in the raw folder, replacing whichever weapon you want.
2) Yeah, the only three parts for moods are feet (sandals), legs (trousers), and torso (robes). But you'll get slaughtered without metal armour if you engage in melee. I recommend staying behind fortifications. Also, the ancient Mesopotamians had metal, just in limited quantities and mostly from trade -- so don't feel culturally obliged to avoid importing metal weapons and armour, at the least.
3) Sorry, you can't ride animals, even with modding. That would be awesome. I think it's coming in a future version, though.
4) No, crowns can only be equipped in adventurer mode or by migrants before they come to your fort as trophies.
I don't know much about the Middle East after Alexander the Great, but before then pretty much everything was made of mud brick (ie, clay). If you have clay, you can get infinite building materials just by having someone repeat gather clay. Useful when you don't want to do intensive mining.