Actually merchants count their total wealth on enter and on exit, so if they leave with no goods (or with less wealth) they think "oh it's a shitty place to trade" and will bring less goods next time. But if they leave with great profit (btw you can give them gifts, donate them a few legendary [20] roasts and they will be ecstatic!) they will bring much more wagons/mules next time they arrive. I ended with 5+ wagons.
So no noticeable difference between a happy and ecstatic dwarf? Good.
Actually there may be or may be not. The game uses numeric scale with 100+ (I may be wrong, use this numbers just to imagine the situation) happiness = happy and 150+ = ecstatic. However 400+ will be still ecstatic, so it's not (-1 unhappy, 3 happy, 4 ecstatic), there may be a great difference which you can't see (without dwarven companion).
"Legendary" level is not single too, there're "Legendary +1", "Legendary +2" etc. but it's not indicated.
B)Will creature jump in a stairless hole 1 Z-level deep (in a hole made to bring some light in my fortress, for example) ?
1. It may jump in if it's crazy/berserking/melancholy. It can be your legendary weaponsmith with broken artifact etc. or mad enemy.
2. The creature may occasionally "dodge" in it if it's fighting near it outside.
3. The fliers will go in. Use grates/glass floors.
E)Why the hell must my nobles be happy? Do they have some kind of super preppy kamehameha? Can I just ignore the useless ones?
Be careful with this.
1. Try to pick a mayor on embark which likes some kind of soil (no items from it) and some simple/cheap materials.
2. There're some useful nobles (well, actually dungeon master).
3. If you're sure that the noble is "useless" or it likes adamantine you'd better kill it right away (or make yourself a challenge). Nobles get friends while they live in your fortress so the death of a single philosopher may lead to a fortress-wide tantrum spiral.
Also note that if some material/item is prohibited for export you can get some real troubles. If the merchants leave the map with such material (even if it's a decoration of some maple earring and it's their own!) the noble will get STACKED bad thoughts for EVERY such item about it being exported and noone to punish (well, it belonged to merchants from the start) and will occasionally throw a tantrum; buy all such things from caravans (which may be problematic if you are at good relations with them, they will bring A LOT of stuff to check... Trade depot dismantling helps).