Refuse management shouldn't be a problem with magma to drop it into.
Dumping into magma can be an issue if you don't dump it from high enough. I had an outbreak of unexplained burns, including an incinerated weaponsmith, before I figured out what I was doing wrong. Magma mist can go 3 tiles up and outward, so make sure to drop from at least 4 Zs above the surface of the magma, like so:
##########
# Dump from here
# ########
# ##
# ##
Magma
You mentioned your forge level is directly above the magma sea. Did you make sure to place the solid tile (the darker green Xs when placing the building) of the smelters and forges above the hole in the floor? There are magma dwelling creatures like fire imps and magma crabs that can attack your dwarves and set your fuel stockpile on fire. Blocking the hole with the unwalkable tile of the workshop prevents this.
If there are no other open access points to the magma sea (like, say, your waste disposal chute), then building destroyers can't hurt the workshops from below. However, it can be hard to absolutely guarantee there is no other access (especially if you have a disposal chute), and due to an oddity in how building destroyers work, if they in theory
could access a building from another angle, they can destroy it from otherwise invalid directions, like from below. This means that if some magma men or fire men show up, or if a forgotten beast or other large nasty manages to get into the magma sea (and is capable of surviving there), it could get in through the forges. For this reason it's usually best to somehow (pumps, channels, or minecarts) move some magma to a secure area for your industries, rather than building directly over the magma sea. However, I'm lazy and usually don't do that.
As for securing a mountain side, as others have said, smoothing the top, making fortifications, or making an overhang should stop climbers coming down. It may be best to put a wall on top of the mountain, blocking access to the cliff, and smooth that instead, to prevent them from coming up to the edge of the cliff and shooting down at your dwarves below.
Don't worry about siege engines. Invaders don't use them, and you shouldn't either, since their useless. If you like the idea of siege engines, and want something similar, I recommend minecarts. You could simply have tracks running down your entrance hall, and crush the enemies with a train of lead minecarts, or you could actually launch the carts at them, or you could crash a filled cart hard against a wall, launching the contents of the cart forward and 1 Z up. If you throw weapons at them, the weapons will actually use the correct attack, rather than just bashing them, so if you train your smiths by having them pump out copper weapons, keep them around to launch at invaders (or put in weapon traps) rather than selling them to caravans. Boulders are also an effective weapon. The most effective weapon is actually water, since during flight it is simulated as a solid (and rather heavy) boulder of water, and liquids can be automatically loaded for reliable operation and a high rate of fire. Magma could be used, but it is much more difficult to set up, and is only marginally more effective.