Why not? The Numenorians were hunting down orcs, not humans. It'd be easy for a single human to slip away in the chaos of their arrival.
The villagers had no knowledge of any force coming to aid them, so being besieged in a keep was slow death even if the orcs didn't immediately break in. They couldn't outlast the orc army, they had to destroy it, and the self-destructing keep was the cheapest way to take out a huge portion of it.
No - that was the whole point of what the orcs have been doing all this time. Digging the tunnels and trenches, not as a means of reaching the villagers with cover from the sun, but as a channel to direct a huge deluge of water directly into the lava chamber.