4) When making obsidian - how much magma and water has to mix? If I have a pool of 7/7 magma, will I need 7/7 water dumped on it to make obsidian?
It's weird. Technically, you only need 1/7 water and 1/7 magma to make obsidian, but in practice, 1/7 water that is dumped directly onto magma steams with no effect. 2/7 water next to 1/7 magma will create obsidian when the 2/7 water "flows" onto the magma square, but then you're left with a leftover 1/7 water square.
Since water tends to be much easier to come by in large quantities, I never pay attention to how much water I'm using. You can make obsidian with as little as 1/7 magma, definitely.
5) Is there a way to check on the value of a room without adding up everything in it one by one?
'r' menu should do this.
6) Any advice on leveling up weapon and armor smithing? Any one item seems to level up a dwarf very slowly, and I can only trade for so many metal bars. Only my masons and children seem to ever get strange moods...
You can melt the stuff you make for 1/3 of the metal back, which helps stretch things a bit. Other than that, you can take peasants, and have them make just enough weapons/armor so that it is their highest craft skill (shouldn't take too long). If they get a fey/secretive mood, legendary weaponsmith or armorer.
You could also test crafting leather armor/wooden shields for armorer, or obsidian swords for weaponsmithing. They might work. I haven't test those, though.