Well, truthfully...I would have had the curb in place before pulling the mud pan. You could custom cut some bricks to fit in there. Adhere them with a modified thinset and coat the whole deal with Hydroban or Redgard. Having said that, what is your water management system? I don't see a liner going up the sides, but the pic doesn't show much. Are you putting Kerdi over everything or what? Not enough info.....

I just looked again at the trap door space, and gave it some more thought. Here's my take: Stain treads a dark shade Match floor color to stain Paint risers white Paint trap door AND bottom stair same as floor Beware! You may need a small colorful rug under the last stair/on floor(hard to see stair at night) For the rails,etc. The most common/best,imo, look is white paint on the rails and stain on post and newel. Oh! New thought re. last stair: Stain it in a light honey and stencil a design in floor color.It will stand out( for safety) and neat effect! Hmmm, you could instead, treat that trap door with that special effect. It's going to be charming, whatever you do.

Floorman here 40 years experience professional installer First. You have a bigger leak under floor if it buckling. Sound to me you would be better to have vinyl plank installed instead of wood. If you want a professional job it will cost you $10 plus for wood and labor no furniture. Labor alone is $5 in Missouri. The cheap hardwood is all going to be soft easy to scratch. Vinyl plank would be cheaper. Labor is $1.50 to $2.00 sq ft. Its glue down or it can be loose laid if you buy that type. If a peace gets scratched easier to replaced even for DIY but that peace will have to be glued. Vinyl plank looks like real wood now days. Water won’t hurt it as long as not a major leak. If your on concrete for wood or vinyl a moisture test should be taken first to see if there’s any hydrostatic pressure if there is it shouldn’t lay either one would have to fix problem first. With vinyl plank if it is glued you might need 1/4” underlayment on sub floor. But loose snap in vinyl plank you shouldn’t need underlayment. If your floor is wavy so will the vinyl plank look. Just my advice.

I've personally sold 6mm cork underlay + 1/2 cork floating floor to a homeowner in NORTHERN Saskatchewan (the Northwest Territories' boarder was only an hour drive away). She put it in her basement. The basement is now the WARMEST spot in the house. The cork flooring and underlay raised the temperature in the basement by 7 C (12 F). It is also the quietest spot in the house. And the cushion on that floor is impressive. The winters in that area = -50C for weeks at a time. If cork+cork can help in this situation...it is going to be warm enough just about anywhere in N.America. If you use cork on cork...you don't have to do anything 'expensive' like a dimple membrane. A $0.06/sf vapour barrier (6mil poly sheeting) + red duct tape (moisture resistant) is all you need. I've sold dozens and dozens of cork + cork flooring products in Canada = cold climate. I've never had anyone come back and say, "The floor is too hard. I can't believe I listened to you." Never. And I've only had one complaint about temperature issues with cork+cork in a basement. The homeowner used 6mm cork underlay + 1/2" cork floating floor. The hubby LOVES it...the wife (mid 50's and going through 'The Change') finds it too HOT. The basement doesn't drop below 21 C (70 F) even with all the vents closed off to the area. That's the ONE complaint I've had about a basement install with cork+cork as the option. It was too WARM. Sigh...I tried to tell hubby to go with the 3mm underlayment...but he thought 'thicker is better' but forgot about how difficult it can be for someone who runs 'too hot' to live in a space that is too warm.