The floor of my wannabe magma forge is made of a non-magma-safe stone. This means, I presume, that I will have to build a magma "irrigation" channels made entirely out of magma-safe one - both the walls AND the floor, I guess.
Natural stone walls and floor (and also constructed walls and floor) are immune to melting. Only movable items (like stones or equipment) or articulating objects (like doors, floodgates, bridges and pumps) will melt in magma.
Next: I want to make my ramp that go down at least two tiles wide; I assume this will allow multiple dwarves to use it at the same time in both direction. What is the best practice in this case? Is three better than two?
Wider means less time lost to collisions between dwarves, but slower pathfinding and lower FPS (Frames Per Second). I just use a 3 x 3 staircase. It's only sixty to one hundred tiles, which takes the same length of time whether it's horizontal travel or vertical travel.
Next: Is there a better way to store bars and wood rather than just one unit per tile? Can I build a container for them?
Furniture, Wood and Stone cannot be stacked in Bins. Bars and Blocks can.