1) I imagine a glacier would be the harder of the two. A desert still has cacti for wood, and sand for glass. Although it is still harder than typical wilderness due to scarcity of water.
2) You can modify the trap components themselves in the raws(raw/objects/item_trapcomp.txt) to be weaker but you can't do the same with cage traps. Weapon traps not using the specialty trap components will also be unaffected.
3) Bolt damage to 1/3 is best. Some also set the damage type to bludgeon. Nothing can be done about the rate of fire. Without limiting yourself to bone or wood crossbows, and that just means they become rapidfire slower.
4) I play a custom race that has the [CARNIVORE] tag. Plants won't feed the fortress. You could just add the tag to dwarves or you could modify plants like plump helmets to grow slower or need to be processed before eaten.
5) creatures with buildingdestroyer can already destroy raised bridges. They just aren't particularly common. As for preventing atom smashing I think a large size prevents that. Nothing you can really do about bridges.
6) Anger also seems to do well with my guys. But it is difficult to tell when nothing goes wrong. For depression high values should work 20:70:100 should be plenty of emo without being too crazy. For cheerfulness lower values should do. 0:30:80. With those you should have a bunch of dwarves who wear black and whine about death all the time, with the occasional sane dwarf.
7) I can't really help ya much there. What I've done is to make new creatures that are near megabeast power that wander the world as normal wildlife. You could do the same, or look through some public mods for creatures like that.
That's all I can think of, may add more in an edit if it comes to me.