I tried to visualize what you have to work with and think you might wall in the furnace and water heater if it's legal and safe. Forget the door to your office.
Give them an upside-down "L" shaped room of their own. Sort of like an upside-down Idaho with a vented door at the Canadian border. Solves all your ventilation issues.
This leaves your darkroom as a right-side up "L" with a large 5 x 9 main work area plus a vestibule.
My first thought was to have the sink against back wall and the door to the left of the sink. That way you can open the door soon as the white lights are on... and it would be convenient to walk out to the laundry tub and wash your trays and tanks there.
If you did that, the vestibule should be a comfortable fit for the enlarger and a dry side shelf. The 9 foot walkway along the sink could be left open or you could put a narrow chemical storage shelf / bookshelf behind where you walk.