When one of my dogs is misbehaving, I don't resort to anything physical, because in my experience, it won't do anything positive. If you catch her in the act, then my response is to raise my voice with a distinctive noise (I use a sound like, "Agh Ahh!") and then when she looks, I say, "NO", and take whatever it is from her. Whatever the word/phrase/action is, just be sure the two of you are consistent, even if it's saying, "OUTSIDE", before you open the door, so that as she gets older, she'll associate that word with going downstairs. I wouldn't use anything physical to reprimand her, but that's just me.
And when there's poop in the corner, unless you catch her in the act, I don't see any value in bringing her to the mess, rubbing her nose in it, and scolding with a stern, "what did you do????" When I first got my basenji puppy 9 yrs ago, I made that mistake. My Lucy looked at me so innocently like, "YUCK! Mom, get me away from this mess!". She didn't seem to connect the smelly stuff with her actions from hours ago. (The look is probably similar to the look you get from Waffles when she is done going up stairs, lol. ) And while you never suggested that you were going to rub her nose in it, I thought I'd let you know about my experience.
You guys are doing great! Congrats on the successes!
Hugs and roos-