Your wagon... spawned.... on... the ice... !?! AWESOME. I bet that was exciting.
I suppose there weren't as many cave-ins as happened on my similar map because the water flowed in from the outside, so most of the obsidianized squares were attached. I had a volcano actually at the bottom of a pre-existing underground lake, so as soon as I unpaused I got 60 or 70 cave-ins from obsidian being created somewhat randomly in the middle of the pipe until it finally grew in from the edges.
The irregularity in the obsidian top is caused by the occasional cave-ins. As you mentioned, obsidian that froze diagonally from any existing obsidian wasn't supported and fell. This caused the magma to splash somewhat (including the generation of deadly magma mist), but it just splashed upward into some more water and froze itself. Most of the squares of obsidian generated in this fashion probably also fell to the bottom of the world, but some no doubt landed on already-frozen obsidian to generate these odd formations.
I'm amazed you only lost three dwarves to drowning. Your two stranded survivors probably managed to swim out onto their isolated island as the ice melted. Untrained dwarves can't really swim that well, but they can climb out if there's a ramp or stairs leading into the water. NOTE: do not assume that this will happen when it freezes again in the fall. It will not. Freezing ice, like obsidian, is instantaneous and obliterates any creatures caught inside when it happens. Keep your dwarves the hell away from the water until it freezes again.
Also, this embark may or may not actually be practical to play. This thaw will happen every year, with the same half-hour freeze in the game each time while it calculates millions of flows. I would find that aggravating after a while.