Last-minute-3am-special hack: intensive fire will now eventually demolish walls. Since floors burn less easily than walls, if you can clear the fire you should have a sustainable alternative path.
How you clear that path is another matter: the player can currently drop 2 water bombs per level, but they’re quite weak compared to the hose floods, and are probably best for coving your back than offense.
Which poses two questions: why would you need to double back, and what tools can I give the player that are more offensive?
The first I can hopefully answer with side objectives - a control panel that activates sprinklers, civilians to rescue, extra equipment, etc.
The second is harder. I want to do an axe for breaking down walls, and maybe hard-to-burn rubble that can be moved to block the way. Neither of those help if the fire is engulfing the stairs though.
I will sleep on it.