Ok... First, let's discuss martial arts.
ANY untrained dwarf or human would by ACCIDENT dodge into the lava. But you don't consider dispersed heat? Humans and dwarves AVOID going in the direction of heat unless sentiently decide to do so. As in dodging into an area you know is dangerous does NOT happen. Even with novices.
Of course a fall is different. Falls, sure. Bug Magma? NO!
Now let's factor in skill. Any person who trained at least a bit in any martial art knows how to keep an eye on his surroundings. Krav Maga has that as a core element. If you can use something as a weapon against your opponent, you will! That can be even making THEM dodge into the magma.
Awareness in combat is NOT as hard as people make it sound.
As for disrupting a fight between two fighters who are totally concentrated, well... They are not going to hit you on purpose. They can easily tell apart you from their enemies. Unless they never seen you before.
That's normally true, but as anyone who's ever been in a STREET fight (ie. potentially a fight for your life) will tell you is that all training can go out the window. Get a room full of Bikers and Black belt in the craziest martial arts you can name, but who've never been in a fight, and my money goes to the Bikers. I've seen people run INTO traffic to get away from a punch to the head, let alone someone swinging a gigantic axe at your head.
The point you about the heat (and even the smell, which would be overwhelming), once your engaged in combat at a point where one false move could send you tumbling into said liquid hot magma, then Murphies Law takes over from there.
Think about it: No one would normally run up to the edge of a pool of magma, unless they where going there for a reason. The reason is "I want to stop this hell beast before it starts burning everything in site" so they run to fight. Stupidly, they run RIGHT to the edge of the pool, draw out a HEAVY, metal axe and swing it around with savage fury. Occasionally they even go berserk, a state in which self preservation is totally forgotten. I'm suprised they don't fall in more often, really, and the image is really epic.
My advice is to station the hand-to-hand fighters near the pool and don't them them roam. When the beasties move a little furthure away from the hellfires of creation, THEN go attack them.